qapi: protect against NULL QObject in qmp_input_get_object

A NULL qobj can occur when a parameter is fetched via qdict_get, but
the parameter is not in the command.  By returning NULL, the caller can
choose whether to raise a missing parameter error, an invalid parameter
type error, or use a default value.  For example, qom-set could can
use this to reset a property to its default value, though at this time
it will fail with "Invalid parameter type".  In any case, anything is
better than crashing!
Reviewed-by: NAnthony Liguori <anthony@codemonkey.ws>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

qapi/qmp-input-visitor.c

@@ -49,10 +49,12 @@ static const QObject *qmp_input_get_object(QmpInputVisitor *qiv,
         qobj = qiv->stack[qiv->nb_stack - 1].obj;
     }
 
-    if (name && qobject_type(qobj) == QTYPE_QDICT) {
-        return qdict_get(qobject_to_qdict(qobj), name);
-    } else if (qiv->nb_stack > 0 && qobject_type(qobj) == QTYPE_QLIST) {
-        return qlist_entry_obj(qiv->stack[qiv->nb_stack - 1].entry);
+    if (qobj) {
+        if (name && qobject_type(qobj) == QTYPE_QDICT) {
+            return qdict_get(qobject_to_qdict(qobj), name);
+        } else if (qiv->nb_stack > 0 && qobject_type(qobj) == QTYPE_QLIST) {
+            return qlist_entry_obj(qiv->stack[qiv->nb_stack - 1].entry);
+        }
     }
 
     return qobj;