avformat/srtdec: skip initial random line breaks.

I found a bunch of (recent) SRT files in the wild with 3 to 10 line
breaks at the beginning.

libavformat/srtdec.c

@@ -37,6 +37,8 @@ static int srt_probe(AVProbeData *p)
     if (AV_RB24(ptr) == 0xEFBBBF)
         ptr += 3;  /* skip UTF-8 BOM */
 
+    while (*ptr == '\r' || *ptr == '\n')
+        ptr++;
     for (i=0; i<2; i++) {
         if ((num == i || num + 1 == i)
             && sscanf(ptr, "%*d:%*2d:%*2d%*1[,.]%*3d --> %*d:%*2d:%*2d%*1[,.]%3d", &v) == 1)