avformat/srtdec: more lenient first line probing

Fixes Ticket #4898
(cherry picked from commit 7218352e02)

libavformat/srtdec.c

@@ -41,9 +41,11 @@ static int srt_probe(AVProbeData *p)
         ff_text_r8(&tr);
 
     /* Check if the first non-empty line is a number. We do not check what the
-     * number is because in practice it can be anything. */
+     * number is because in practice it can be anything.
+     * Also, that number can be followed by random garbage, so we can not
+     * unfortunately check that we only have a number. */
     if (ff_subtitles_read_line(&tr, buf, sizeof(buf)) < 0 ||
-        strtol(buf, &pbuf, 10) < 0 || *pbuf)
+        strtol(buf, &pbuf, 10) < 0)
         return 0;
 
     /* Check if the next line matches a SRT timestamp */