avcodec/dvdsubdec: Sanity check len in decode_rle()

Fixes: Timeout
Fixes: 9778/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DVDSUB_fuzzer-5186007132536832

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit e7b023e1db)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dvdsubdec.c

@@ -124,6 +124,8 @@ static int decode_rle(uint8_t *bitmap, int linesize, int w, int h,
             len = decode_run_8bit(&gb, &color);
         else
             len = decode_run_2bit(&gb, &color);
+        if (len != INT_MAX && len > w - x)
+            return AVERROR_INVALIDDATA;
         len = FFMIN(len, w - x);
         memset(d + x, color, len);
         x += len;