avcodec/pngdec: Check IHDR/IDAT order

Fixes out of array access
Fixes: asan_heap-oob_20a6c26_2690_cov_3434532168_mail.png
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 79ceaf827b)

Conflicts:

	libavcodec/pngdec.c

libavcodec/pngdec.c

@@ -582,6 +582,12 @@ static int decode_frame(AVCodecContext *avctx,
         case MKTAG('I', 'H', 'D', 'R'):
             if (length != 13)
                 goto fail;
+
+            if (s->state & PNG_IDAT) {
+                av_log(avctx, AV_LOG_ERROR, "IHDR after IDAT\n");
+                goto fail;
+            }
+
             s->width  = bytestream2_get_be32(&s->gb);
             s->height = bytestream2_get_be32(&s->gb);
             if (av_image_check_size(s->width, s->height, 0, avctx)) {