Avoid a harmless potential integer overflow in png_XYZ_from_xy().

This change replicates commit d46b57 applied to the libpng16
codebase.

Bug: 21039636
(cherry picked from commit 771583a01bd0756a1b6861ab1f00b7bd1842a0cd)

Change-Id: I20bc002b18d14279d7fb407368077487c65f1ff4
1 file changed