concatdec: Do not pass NULL to memcmp

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavformat/concatdec.c

@@ -120,7 +120,7 @@ static int add_file(AVFormatContext *avf, char *filename, ConcatFile **rfile,
 
     proto = avio_find_protocol_name(filename);
     proto_len = proto ? strlen(proto) : 0;
-    if (!memcmp(filename, proto, proto_len) &&
+    if (proto && !memcmp(filename, proto, proto_len) &&
         (filename[proto_len] == ':' || filename[proto_len] == ',')) {
         url = filename;
         filename = NULL;