qdict: Make qdict_extract_subqdict() accept dst = NULL

This function extracts all options from a QDict starting with a
certain prefix and puts them in a new QDict.

We'll have a couple of cases where we simply want to discard those
options instead of copying them, and that's what this patch does.
Signed-off-by: NAlberto Garcia <berto@igalia.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

qobject/block-qdict.c

@@ -158,20 +158,25 @@ void qdict_flatten(QDict *qdict)
     qdict_flatten_qdict(qdict, qdict, NULL);
 }
 
-/* extract all the src QDict entries starting by start into dst */
+/* extract all the src QDict entries starting by start into dst.
+ * If dst is NULL then the entries are simply removed from src. */
 void qdict_extract_subqdict(QDict *src, QDict **dst, const char *start)
 
 {
     const QDictEntry *entry, *next;
     const char *p;
 
-    *dst = qdict_new();
+    if (dst) {
+        *dst = qdict_new();
+    }
     entry = qdict_first(src);
 
     while (entry != NULL) {
         next = qdict_next(src, entry);
         if (strstart(entry->key, start, &p)) {
-            qdict_put_obj(*dst, p, qobject_ref(entry->value));
+            if (dst) {
+                qdict_put_obj(*dst, p, qobject_ref(entry->value));
+            }
             qdict_del(src, entry->key);
         }
         entry = next;