Set prof_ident to NULL after free

This caused the bug mentioned in the PR comment:

```
It seems with the changes done here we get a crash in: src/xmpp/message.c message_handlers_init() when looking up handlers: ProfMessageHandler *handler = g_hash_table_lookup(pubsub_event_handlers, curr->data);.

Steps to reproduce:

    open Profanity and connect
    /autoping set 10
    /autoping timeout 10
    stop WiFi/connection
    wait for Lost connection
    restart wifi
    /connect
```

src/xmpp/connection.c

@@ -247,6 +247,7 @@ connection_disconnect(void)
     }
 
     free(prof_identifier);
+    prof_identifier = NULL;
 }
 
 void
@@ -681,7 +682,7 @@ static void _compute_identifier(const char *barejid)
     assert(b64 != NULL);
     g_free(hmac);
 
-    //in case of reconnect
+    //in case of reconnect (lost connection)
     free(prof_identifier);
 
     prof_identifier = b64;