avformat/avidec: Limit formats in gab2 to srt and ass/ssa

This prevents part of one exploit leading to an information leak

Found-by: Emil Lerner and Pavel Cheremushkin
Reported-by: Thierry Foucu <tfoucu@google.com>

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit a5d849b149)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/avidec.c

@@ -1088,6 +1088,9 @@ static int read_gab2_sub(AVFormatContext *s, AVStream *st, AVPacket *pkt)
         if (!sub_demuxer)
             goto error;
 
+        if (strcmp(sub_demuxer->name, "srt") && strcmp(sub_demuxer->name, "ass"))
+            goto error;
+
         if (!(ast->sub_ctx = avformat_alloc_context()))
             goto error;