lavf: don't segfault when a NULL filename is passed to avformat_open_input()

This can easily happen when the caller is using a custom AVIOContext.

Behave as if the filename was an empty string in this case.

CC: libav-stable@libav.org
(cherry picked from commit a5db8e4a1a)

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavformat/utils.c

@@ -634,7 +634,7 @@ int avformat_open_input(AVFormatContext **ps, const char *filename, AVInputForma
     }
 
     s->duration = s->start_time = AV_NOPTS_VALUE;
-    av_strlcpy(s->filename, filename, sizeof(s->filename));
+    av_strlcpy(s->filename, filename ? filename : "", sizeof(s->filename));
 
     /* allocate private data */
     if (s->iformat->priv_data_size > 0) {