handler: shut compiler up

Fixes the next warning:
    src/handler.c:683:25: error: ‘key2’ may be used uninitialized in
    this function [-Werror=uninitialized]

In fact key2 initialised and used under the same condition.

src/handler.c

@@ -627,7 +627,7 @@ void handler_system_delete_all(xmpp_conn_t *conn)
     xmpp_handlist_t *item, *next, *head, *head_old;
     hash_iterator_t *iter;
     const char *key;
-    char *key2;
+    char *key2 = NULL;
 
     /* TODO unify all kinds of handlers and avoid copy-paste below */