qdict: Make conversion from QObject * accept null

qobject_to_qdict() crashes on null, which is a trap for the unwary.
Return null instead, and simplify a few callers.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <1444918537-18107-4-git-send-email-armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NLuiz Capitulino <lcapitulino@redhat.com>

qga/main.c

@@ -573,7 +573,6 @@ static void process_command(GAState *s, QDict *req)
 static void process_event(JSONMessageParser *parser, QList *tokens)
 {
     GAState *s = container_of(parser, GAState, parser);
-    QObject *obj;
     QDict *qdict;
     Error *err = NULL;
     int ret;
@@ -581,9 +580,9 @@ static void process_event(JSONMessageParser *parser, QList *tokens)
     g_assert(s && parser);
 
     g_debug("process_event: called");
-    obj = json_parser_parse_err(tokens, NULL, &err);
-    if (err || !obj || qobject_type(obj) != QTYPE_QDICT) {
-        qobject_decref(obj);
+    qdict = qobject_to_qdict(json_parser_parse_err(tokens, NULL, &err));
+    if (err || !qdict) {
+        QDECREF(qdict);
         qdict = qdict_new();
         if (!err) {
             g_warning("failed to parse event: unknown error");
@@ -593,12 +592,8 @@ static void process_event(JSONMessageParser *parser, QList *tokens)
         }
         qdict_put_obj(qdict, "error", qmp_build_error_object(err));
         error_free(err);
-    } else {
-        qdict = qobject_to_qdict(obj);
     }
 
-    g_assert(qdict);
-
     /* handle host->guest commands */
     if (qdict_haskey(qdict, "execute")) {
         process_command(s, qdict);

qobject/qdict.c

@@ -46,9 +46,9 @@ QDict *qdict_new(void)
  */
 QDict *qobject_to_qdict(const QObject *obj)
 {
-    if (qobject_type(obj) != QTYPE_QDICT)
+    if (!obj || qobject_type(obj) != QTYPE_QDICT) {
         return NULL;
-
+    }
     return container_of(obj, QDict, base);
 }
 
@@ -269,7 +269,7 @@ QList *qdict_get_qlist(const QDict *qdict, const char *key)
  */
 QDict *qdict_get_qdict(const QDict *qdict, const char *key)
 {
-    return qobject_to_qdict(qdict_get_obj(qdict, key, QTYPE_QDICT));
+    return qobject_to_qdict(qdict_get(qdict, key));
 }
 
 /**