(trivial) NULL protection for ScreenRec->DestroyPixmap and dixDestroyPixmap()
Add NULL protection for calls to ScreenRec->DestroyPixmap() as well as inside dixDestroyPixmap
Right now, we're assuming that even when deep nesting involved, the proc vector is always set to a valid function. One the one hand it requires extra dummy procs in some cases, OTOH it's making upcoming refactoring of the code flow unnecessarily complex.
The big plot (of subsequent commits) is splitting out the extension's (and possibly subsystem's) special logic out of the wrapping chain and let them be executed independently from the DDX/drivers - when applicable even only when the pixmap is really destroyed (not just unref'ed). (At some later point, it might even become be actually a valid situation that DestroyPixmap vector really being NULL.)