Don't unload OMEMO cryto on disconnect if OMEMO wasn't loaded

If OMEMO wasn't loaded, removing key would create empty identity.txt
file. Then at load time it would segfault profanity.

Another commit should fix load of malformed identity.txt.

src/omemo/omemo.c

@@ -241,6 +241,10 @@ omemo_on_connect(ProfAccount *account)
 void
 omemo_on_disconnect(void)
 {
+    if (!loaded) {
+        return;
+    }
+
     signal_protocol_signed_pre_key_remove_key(omemo_ctx.store, omemo_ctx.signed_pre_key_id);
     _g_hash_table_free(omemo_ctx.signed_pre_key_store);