dirac_parser: check prev_pu_offset before using it

Fixes out of array read

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dirac_parser.c

@@ -161,7 +161,9 @@ static int dirac_combine_frame(AVCodecParserContext *s, AVCodecContext *avctx,
          * we can be pretty sure that we have a valid parse unit */
         if (!unpack_parse_unit(&pu1, pc, pc->index - 13)                     ||
             !unpack_parse_unit(&pu, pc, pc->index - 13 - pu1.prev_pu_offset) ||
-            pu.next_pu_offset != pu1.prev_pu_offset) {
+            pu.next_pu_offset != pu1.prev_pu_offset                          ||
+            pc->index < pc->dirac_unit_size + 13LL + pu1.prev_pu_offset
+        ) {
             pc->index -= 9;
             *buf_size = next-9;
             pc->header_bytes_needed = 9;