Fix a division by zero when processing malformed BMP files.

This fixes a division by 0 when processing a maliciously crafted BMP
file. No impact beyond DoS.

Backport of 661f6bfd032dacc62841037732816a583640e187

Task-number: QTBUG-44547
Change-Id: I43f06e752b11cb50669101460902a82b885ae618
Reviewed-by: Thiago Macieira <[email protected]>

Author: richmoore

src/gui/image/qbmphandler.cpp

@@ -319,10 +319,16 @@ static bool read_dib_body(QDataStream &s, const BMP_INFOHDR &bi, int offset, int
         }
     } else if (comp == BMP_BITFIELDS && (nbits == 16 || nbits == 32)) {
         red_shift = calc_shift(red_mask);
+        if (((red_mask >> red_shift) + 1) == 0)
+            return false;
         red_scale = 256 / ((red_mask >> red_shift) + 1);
         green_shift = calc_shift(green_mask);
+        if (((green_mask >> green_shift) + 1) == 0)
+            return false;
         green_scale = 256 / ((green_mask >> green_shift) + 1);
         blue_shift = calc_shift(blue_mask);
+        if (((blue_mask >> blue_shift) + 1) == 0)
+            return false;
         blue_scale = 256 / ((blue_mask >> blue_shift) + 1);
     } else if (comp == BMP_RGB && (nbits == 24 || nbits == 32)) {
         blue_mask = 0x000000ff;