[libpng14] Avoid out-of-bounds memory access in png_user_version_check().

Simplified and future-proofed png_user_version_check().
 [libpng14] Imported from libpng-1.4.14beta01.tar
 [libpng14] Imported from libpng-1.4.14rc01.tar
[libpng14] Added "-D_CRT_SECURE_NO_WARNINGS" to CFLAGS in

scripts/makefile.vcwin32.
 [libpng14] Imported from libpng-1.4.14rc02.tar
 [libpng14] Imported from libpng-1.4.14.tar