[libpng16] Revise man page to demonstrate dealing with unknown chunk

handling in libpng.1.5.x and with bugs in libpng-1.4.x and earlier.
[libpng16] Avoid dereferencing NULL pointer possibly returned from

png_create_write_struct() (Andrew Church).
[libpng16] Imported from libpng-1.6.3beta03.tar
[libpng16] Bump version to 1.6.3beta04