avformat/concatdec: Check file

Fixes: null pointer dereference
Fixes: -stream_loop 1 -ss 00:00:05 -i zgclab/ffmpeg_crash/poc2 -codec:v copy -codec:a aac -y output.mp4

Found-by: Wang Dawei and Zhou Geng, from Zhongguancun Laboratory
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit a5d1497f33)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/concatdec.c

@@ -638,6 +638,11 @@ static int concat_parse_script(AVFormatContext *avf)
         }
     }
 
+    if (!file) {
+        ret = AVERROR_INVALIDDATA;
+        goto fail;
+    }
+
     if (file->inpoint != AV_NOPTS_VALUE && file->outpoint != AV_NOPTS_VALUE) {
         if (file->inpoint  > file->outpoint ||
             file->outpoint - (uint64_t)file->inpoint > INT64_MAX)