What is the correct format for final block in EVP_DecryptFinal_ex for decryption?

I have implemented a simple AES-256-GCM encryption and decryption for learning purposes. While testing my code if I enter strings lengths multiples of 6 then I get the correct output but for other cases the decrypted data has some garbage characters appended to it.

Case1:
Enter string: abcdef
Enter key: sdasdasdsa
-^%�
abcdef
6

Case2:
Enter string: abcdefghi
Enter key: sadsadsad
򈇢\h�,�[�
abcdefghi�\�
-1

Now I read on the http://ift.tt/148k9Kg that

EVP_DecryptFinal() will return an error code if padding is enabled and the
final block is not correctly formatted.

But since padding is enabled by default in this case I am guessing that the problem is with the correct formatting of the final block. I need this urgently, so please help me figure out where am I going wrong. I have attached my code below:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <openssl/evp.h>

void handleErrors()
{
    printf("Some error occured\n");
}

int encrypt(unsigned char *plaintext, int plaintext_len, unsigned char *aad,
    int aad_len, unsigned char *key, unsigned char *iv,
    unsigned char *ciphertext, unsigned char *tag)
{
    EVP_CIPHER_CTX *ctx;

    int len, ciphertext_len=0;

    /* Create and initialise the context */
    if(!(ctx = EVP_CIPHER_CTX_new()))
        handleErrors();

    /* Initialise the encryption operation. */
    if(1 != EVP_EncryptInit_ex(ctx, EVP_aes_256_gcm(), NULL, NULL, NULL))
        handleErrors();

    /* Set IV length if default 12 bytes (96 bits) is not appropriate */
    if(1 != EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_IVLEN, 16, NULL))
        handleErrors();

    /* Initialise key and IV */
    if(1 != EVP_EncryptInit_ex(ctx, NULL, NULL, key, iv)) handleErrors();

    /* Provide any AAD data. This can be called zero or more times as
     * required
     */
    if(1 != EVP_EncryptUpdate(ctx, NULL, &len, aad, aad_len))
        handleErrors();

    /* Provide the message to be encrypted, and obtain the encrypted output.
     * EVP_EncryptUpdate can be called multiple times if necessary
     */
    if(1 != EVP_EncryptUpdate(ctx, ciphertext, &len, plaintext, plaintext_len))
        handleErrors();
    ciphertext_len+= len;

    /* Finalise the encryption. Normally ciphertext bytes may be written at
     * this stage, but this does not occur in GCM mode
     */
    if(1 != EVP_EncryptFinal_ex(ctx, ciphertext + len, &len)) handleErrors();
    ciphertext_len += len;

    /* Get the tag */
    if(1 != EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_GET_TAG, 16, tag))
        handleErrors();

    /* Clean up */
    EVP_CIPHER_CTX_free(ctx);

    return ciphertext_len;
}


int decrypt(unsigned char *ciphertext, int ciphertext_len, unsigned char *aad,
    int aad_len, unsigned char *tag, unsigned char *key, unsigned char *iv,
    unsigned char *plaintext)
{
    EVP_CIPHER_CTX *ctx;
    int len, plaintext_len=0, ret;

    /* Create and initialise the context */
    if(!(ctx = EVP_CIPHER_CTX_new())) 
        handleErrors();

    /* Initialise the decryption operation. */
    if(!EVP_DecryptInit_ex(ctx, EVP_aes_256_gcm(), NULL, NULL, NULL))
        handleErrors();

    /* Set IV length. Not necessary if this is 12 bytes (96 bits) */
    if(!EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_IVLEN, 16, NULL))
        handleErrors();

    /* Initialise key and IV */
    if(!EVP_DecryptInit_ex(ctx, NULL, NULL, key, iv)) handleErrors();

    /* Provide any AAD data. This can be called zero or more times as
     * required
     */
    if(!EVP_DecryptUpdate(ctx, NULL, &len, aad, aad_len))
        handleErrors();

    /* Provide the message to be decrypted, and obtain the plaintext output.
     * EVP_DecryptUpdate can be called multiple times if necessary
     */
    if(!EVP_DecryptUpdate(ctx, plaintext, &len, ciphertext, ciphertext_len))
        handleErrors();
    plaintext_len+= len;

    /* Set expected tag value. Works in OpenSSL 1.0.1d and later */
    if(!EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_TAG, 16, tag))
        handleErrors();

    /* Finalise the decryption. A positive return value indicates success,
     * anything else is a failure - the plaintext is not trustworthy.
     */
    ret = EVP_DecryptFinal_ex(ctx, plaintext + len, &len);

    /* Clean up */
    EVP_CIPHER_CTX_free(ctx);

    if(ret > 0)
    {
        /* Success */
        plaintext_len += len;
        return plaintext_len;
    }
    else
    {
        /* Verify failed */
        return -1;
    }
}


int main (void)
{
    unsigned char str[1024],key[10],ciphertext[1024+EVP_MAX_BLOCK_LENGTH],tag[100],pt[1024];
    unsigned char iv[]="1234567890abcdef";
    unsigned char aad[]="1234567890123456";
    int k;
    printf("Enter string: ");
    scanf("%s",str);
    printf("Enter key: ");
    scanf("%s",key);

    encrypt(str, strlen(str), aad, strlen(aad), key, iv, ciphertext, tag);
    printf("%s\n",ciphertext);
    k = decrypt(ciphertext, strlen(ciphertext), aad, strlen(aad), tag, key, iv, pt);
    printf("%s\n%d\n",pt,k);
}