fix: Incorrect omemo decryption error for Key Transport Messages

Correctly handle OMEMO stanzas that contain a <header> but no <payload>.
These "Key Transport Messages" (heartbeats) are used by some clients
for session maintenance and establishing trust.

Ref: a2726b6a7 made the <payload> element mandatory in the parser.
Ref: 4d49c2b74 the bug became user-visible.

Fixes: https://github.com/profanity-im/profanity/issues/2129
Signed-off-by: Michael Vetter <jubalh@iodoru.org>

src/omemo/omemo.c

@@ -1145,6 +1145,11 @@ omemo_on_message_recv(const char* const from_jid, uint32_t sid,
         return NULL;
     }
 
+    if (payload == NULL) {
+        signal_buffer_free(plaintext_key);
+        return NULL;
+    }
+
     size_t plaintext_len = payload_len;
     plaintext = malloc(plaintext_len + 1);
     res = aes128gcm_decrypt(plaintext, &plaintext_len, payload, payload_len, iv,