avformat: don't crash API users when demuxing mp4

This code is one big chunk of WTF.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/utils.c

@@ -1788,7 +1788,7 @@ void ff_configure_buffers_for_index(AVFormatContext *s, int64_t time_tolerance)
     //We could use URLProtocol flags here but as many user applications do not use URLProtocols this would be unreliable
     const char *proto = avio_find_protocol_name(s->filename);
 
-    if (!(strcmp(proto, "file") && strcmp(proto, "pipe") && strcmp(proto, "cache")))
+    if (proto && !(strcmp(proto, "file") && strcmp(proto, "pipe") && strcmp(proto, "cache")))
         return;
 
     for (ist1 = 0; ist1 < s->nb_streams; ist1++) {