avcodec/dvdsubdec: fix out of bounds accesses

The code blindly trusted buffer offsets read from the file in the RLE
decoder. Explicitly check the offset. Also error out on other RLE
decoding errors.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dvdsubdec.c

@@ -108,6 +108,9 @@ static int decode_rle(uint8_t *bitmap, int linesize, int w, int h,
     int x, y, len, color;
     uint8_t *d;
 
+    if (start >= buf_size)
+        return -1;
+
     bit_len = (buf_size - start) * 8;
     init_get_bits(&gb, buf + start, bit_len);
 
@@ -359,10 +362,12 @@ static int decode_dvd_subtitles(DVDSubContext *ctx, AVSubtitle *sub_header,
                 sub_header->rects[0] = av_mallocz(sizeof(AVSubtitleRect));
                 sub_header->num_rects = 1;
                 sub_header->rects[0]->pict.data[0] = bitmap;
-                decode_rle(bitmap, w * 2, w, (h + 1) / 2,
-                           buf, offset1, buf_size, is_8bit);
-                decode_rle(bitmap + w, w * 2, w, h / 2,
-                           buf, offset2, buf_size, is_8bit);
+                if (decode_rle(bitmap, w * 2, w, (h + 1) / 2,
+                               buf, offset1, buf_size, is_8bit) < 0)
+                    goto fail;
+                if (decode_rle(bitmap + w, w * 2, w, h / 2,
+                               buf, offset2, buf_size, is_8bit) < 0)
+                    goto fail;
                 sub_header->rects[0]->pict.data[1] = av_mallocz(AVPALETTE_SIZE);
                 if (is_8bit) {
                     if (!yuv_palette)