avcodec/pngdec: Check bits per pixel before setting monoblack pixel format

Fixes out of array accesses
Fixes: asan_heap-oob_14dbfcf_4_asan_heap-oob_1ce5767_179_add_method_small.png

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 3e2b745020)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/pngdec.c

@@ -650,7 +650,7 @@ static int decode_frame(AVCodecContext *avctx,
                 } else if ((s->bits_per_pixel == 1 || s->bits_per_pixel == 2 || s->bits_per_pixel == 4 || s->bits_per_pixel == 8) &&
                            s->color_type == PNG_COLOR_TYPE_PALETTE) {
                     avctx->pix_fmt = AV_PIX_FMT_PAL8;
-                } else if (s->bit_depth == 1) {
+                } else if (s->bit_depth == 1 && s->bits_per_pixel == 1) {
                     avctx->pix_fmt = AV_PIX_FMT_MONOBLACK;
                 } else if (s->bit_depth == 8 &&
                            s->color_type == PNG_COLOR_TYPE_GRAY_ALPHA) {