Avoid a harmless potential integer overflow in png_XYZ_from_xy().

This change replicates commit d46b57 applied to the libpng16
codebase.

Bug: 21039636
Change-Id: I1333196b8673aac6552cd438fd580dbd1d60ef11
(cherry picked from commit 771583a01bd0756a1b6861ab1f00b7bd1842a0cd)
1 file changed