Fix segfault when aesgcm url isn't the expected size

Fixes the bug mentioned in
https://github.com/profanity-im/profanity/issues/1478#issuecomment-794161606

The rest of https://github.com/profanity-im/profanity/issues/1478 I
can't reproduce. Seems to work fine.

src/omemo/omemo.c

@@ -1862,6 +1862,7 @@ omemo_parse_aesgcm_url(const char* aesgcm_url,
     }
 
     if (strlen(*fragment) != AESGCM_URL_NONCE_LEN + AESGCM_URL_KEY_LEN) {
+        ret = 1;
         goto out;
     }