avformat/concat: Check protocol prefix

Reviewed-by: Andreas Cadhalpun <andreas.cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 8e32d01432)

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/concat.c

@@ -64,7 +64,10 @@ static av_cold int concat_open(URLContext *h, const char *uri, int flags)
     struct concat_data  *data = h->priv_data;
     struct concat_nodes *nodes;
 
-    av_strstart(uri, "concat:", &uri);
+    if (!av_strstart(uri, "concat:", &uri)) {
+        av_log(h, AV_LOG_ERROR, "URL %s lacks prefix\n", uri);
+        return AVERROR(EINVAL);
+    }
 
     for (i = 0, len = 1; uri[i]; i++)
         if (uri[i] == *AV_CAT_SEPARATOR)