The length checking code validates PutImage height and byte width by
 making sure that byte-width >= INT32_MAX / height. If height is zero,
 this generates a divide by zero exception. Allow zero height requests
 explicitly, bypassing the INT32_MAX check.

 Fix for regression introduced by fix for CVE-2014-8092.

 v2: backports to nx-libs 3.6.x (Mike Gabriel)
Signed-off-by: Keith Packard <keithp@keithp.com>