Free key on error in omemo_start_device_session_handle_bundle()

Fix:
```
==20561== 32 bytes in 1 blocks are definitely lost in loss record 1,467
of 3,678
==20561==    at 0x483677F: malloc (in
/usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==20561==    by 0x4B16C9: omemo_start_device_session_handle_bundle
(omemo.c:167)
==20561==    by 0x43405E: _iq_handler (iq.c:214)
==20561==    by 0x5AF118E: ??? (in /usr/lib64/libmesode.so.0.0.0)
==20561==    by 0x5AEDBDA: ??? (in /usr/lib64/libmesode.so.0.0.0)
==20561==    by 0x5AFA43E: ??? (in /usr/lib64/libmesode.so.0.0.0)
==20561==    by 0x6818AA4: ??? (in /usr/lib64/libexpat.so.1.6.8)
==20561==    by 0x681A3AB: ??? (in /usr/lib64/libexpat.so.1.6.8)
==20561==    by 0x681D7EB: XML_ParseBuffer (in
/usr/lib64/libexpat.so.1.6.8)
==20561==    by 0x5AF0A63: xmpp_run_once (in
/usr/lib64/libmesode.so.0.0.0)
==20561==    by 0x432E5D: connection_check_events (connection.c:104)
==20561==    by 0x4323B3: session_process_events (session.c:255)
==20561==    by 0x42C097: prof_run (profanity.c:128)
==20561==    by 0x4B260D: main (main.c:172)
```

1 files changed, 5 insertions, 0 deletions

diff --git a/src/xmpp/omemo.c b/src/xmpp/omemo.c
index e0f2a70d..e44cc00e 100644
--- a/src/xmpp/omemo.c
+++ b/src/xmpp/omemo.c
@@ -168,13 +168,18 @@ omemo_start_device_session_handle_bundle(xmpp_stanza_t *const stanza, void *cons
 
         const char *prekey_id_text = xmpp_stanza_get_attribute(prekey, "preKeyId");
         if (!prekey_id_text) {
+            omemo_key_free(key);
             goto out;
         }
+
         key->id = strtoul(prekey_id_text, NULL, 10);
         xmpp_stanza_t *prekey_text = xmpp_stanza_get_children(prekey);
+
         if (!prekey_text) {
+            omemo_key_free(key);
             goto out;
         }
+
         char *prekey_b64 = xmpp_stanza_get_text(prekey_text);
         key->data = g_base64_decode(prekey_b64, &key->length);
         free(prekey_b64);