Avoid undefined behavior for removing elements that were not in the tree.

Originally committed as revision 15368 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/tree.c

@@ -119,8 +119,11 @@ void *av_tree_insert(AVTreeNode **tp, void *key, int (*cmp)(void *key, const voi
         return ret;
     }else{
         *tp= *next; *next= NULL;
-        (*tp)->elem= key;
-        return NULL;
+        if(*tp){
+            (*tp)->elem= key;
+            return NULL;
+        }else
+            return key;
     }
 }
 
@@ -188,8 +191,7 @@ int main(void){
         av_tree_insert(&root, (void*)(j+1), cmp, &node);
 
         j= (random()%86294);
-        k= av_tree_find(root, (void*)(j+1), cmp, NULL);
-        if(k){
+        {
             AVTreeNode *node2=NULL;
             av_log(NULL, AV_LOG_ERROR, "removing %4d\n", j);
             av_tree_insert(&root, (void*)(j+1), cmp, &node2);