pngdec: check bits_per_pixel for palette mode.

This fixes a out of array read

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

libavcodec/pngdec.c

@@ -494,7 +494,8 @@ static int decode_frame(AVCodecContext *avctx,
                 } else if (s->bit_depth == 16 &&
                            s->color_type == PNG_COLOR_TYPE_RGB_ALPHA) {
                     avctx->pix_fmt = PIX_FMT_RGBA64BE;
-                } else if (s->color_type == PNG_COLOR_TYPE_PALETTE) {
+                } 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 = PIX_FMT_PAL8;
                 } else if (s->bit_depth == 1) {
                     avctx->pix_fmt = PIX_FMT_MONOBLACK;