pngdec: don't use AV_PIX_FMT_MONOBLACK for apng

AV_PIX_FMT_MONOBLACK has the AV_PIX_FMT_FLAG_BITSTREAM flag, i.e.
linesize can be smaller than width.

Since x_offset is only check against the width, this can lead to
x_offset * bpp >= image_linesize.

In this case ptr could be set to a position outside the image_buf in
png_handle_row, leading to memory corruption and thus crashes.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
