[Bug 48927] Heap buffer underflow in TiffFrameDecode_ReadTile when decoding 1x1 4bpp RGBA image

[WineHQ Bugzilla]

https://bugs.winehq.org/show_bug.cgi?id=48927

--- Comment #4 from Dmitry Timoshkov  ---
I don't have a TIFF image in an appropriate format to test at the moment,
however does the following implementation fix the heap corruption?

/* 1 source byte expands to 2 BGRA samples */
        count = This->decode_info.tile_width * This->decode_info.tile_height;

src = This->cached_tile + count / 2 - 1;
        dst = This->cached_tile + This->decode_info.tile_size;

while (count >= 2)
        {
            BYTE b = *src--;

dst -= 8;
            dst[2] = (b & 0x80) ? 0xff : 0; /* R */
            dst[1] = (b & 0x40) ? 0xff : 0; /* G */
            dst[0] = (b & 0x20) ? 0xff : 0; /* B */
            dst[3] = (b & 0x10) ? 0xff : 0; /* A */
            dst[6] = (b & 0x08) ? 0xff : 0; /* R */
            dst[5] = (b & 0x04) ? 0xff : 0; /* G */
            dst[4] = (b & 0x02) ? 0xff : 0; /* B */
            dst[7] = (b & 0x01) ? 0xff : 0; /* A */

count -= 2;
        }
        if (count)
        {
            BYTE b = *src--;

dst -= 4;
            dst[2] = (b & 0x80) ? 0xff : 0; /* R */
            dst[1] = (b & 0x40) ? 0xff : 0; /* G */
            dst[0] = (b & 0x20) ? 0xff : 0; /* B */
            dst[3] = (b & 0x10) ? 0xff : 0; /* A */
        }