Add support for 12 bytes IV

16 bytes IV should be used. Some clients can't use it so we should also
support decrypting 12 bytes IV.

Fix #1253

src/omemo/crypto.h

@@ -178,5 +178,5 @@ int aes128gcm_encrypt(unsigned char *ciphertext, size_t *ciphertext_len,
 
 int aes128gcm_decrypt(unsigned char *plaintext,
     size_t *plaintext_len, const unsigned char *const ciphertext,
-    size_t ciphertext_len, const unsigned char *const iv,
+    size_t ciphertext_len, const unsigned char *const iv, size_t iv_len,
     const unsigned char *const key, const unsigned char *const tag);