avformat/url: check the result of the strrchr

because it need be check for success, is should not
change the old way if it failure.
fix ticket: 8674
Signed-off-by: Steven Liu <liuqi05@kuaishou.com>

libavformat/url.c

@@ -182,7 +182,7 @@ void ff_make_absolute_url(char *buf, int size, const char *base,
 
     /* Remove the file name from the base url */
     sep = strrchr(buf, '/');
-    if (sep <= root)
+    if (sep && sep <= root)
         sep = root;
 
     if (sep)