avformat/dashdec: check the root url length

if the length of the root url is 0, unnecessary process the root_url

Signed-off-by: Steven Liu <lq@chinaffmpeg.org>
Signed-off-by: liuqi05 <liuqi05@kuaishou.com>

libavformat/dashdec.c

@@ -771,7 +771,7 @@ static int resolve_content_path(AVFormatContext *s, const char *url, int *max_ur
     size = strlen(root_url);
     isRootHttp = ishttp(root_url);
 
-    if (root_url[size - 1] != token) {
+    if (size > 0 && root_url[size - 1] != token) {
         av_strlcat(root_url, "/", size + 2);
         size += 2;
     }