Themes with specified gap images but no actual file crash.

https://bugzilla.gnome.org/show_bug.cgi?id=712536
pixbuf-engine: Improve ThemePixbuf clean up functions

Make theme_pixbuf_destroy() NULL-safe like g_free(), and add a clear
function in the spirit of the g_clear_* family of functions.

https://bugzilla.gnome.org/show_bug.cgi?id=712536
pixbuf-engine: Clean up error conditions and destructors

Simplify the error checks and move all common behaviour into a utility
function.

https://bugzilla.gnome.org/show_bug.cgi?id=712536