avcodec/dnxhd_parser: Fix parser when input does not have nicely sized packets

Fixes: out of array access
Fixes: 15522/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DNXHD_fuzzer-5747756078989312

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 2d900d8fe0)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dnxhd_parser.c

@@ -79,8 +79,9 @@ static int dnxhd_find_frame_end(DNXHDParserContext *dctx,
                     if (remaining <= 0)
                         continue;
                 }
+                remaining += i - 47;
                 dctx->remaining = remaining;
-                if (buf_size - i + 47 >= dctx->remaining) {
+                if (buf_size >= dctx->remaining) {
                     pc->frame_start_found = 0;
                     pc->state64 = -1;
                     dctx->cur_byte = 0;