Monitor: Fix command execution regression

Function is_async_return() added by commit 940cc30d assumes
that 'data', which is returned by handlers, is always a QDict.

This is not true, as QLists can also be returned, in this case
we'll get a segfault.

Fix that by checking if 'data' is a QDict.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

monitor.c

@@ -3700,7 +3700,11 @@ static void monitor_print_error(Monitor *mon)
 
 static int is_async_return(const QObject *data)
 {
-    return data && qdict_haskey(qobject_to_qdict(data), "__mon_async");
+    if (data && qobject_type(data) == QTYPE_QDICT) {
+        return qdict_haskey(qobject_to_qdict(data), "__mon_async");
+    }
+
+    return 0;
 }
 
 static void monitor_call_handler(Monitor *mon, const mon_cmd_t *cmd,