Prevent the -wm command line option from causing a SEGV

The -wm (when mapped) option for the BackingStore support has been
causing the server to dereference a NULL pointer.

This has probably been the case since backing store has been
implemented on top of Composite.

It looks like (some of?) Composite didn’t expect its WIndowPtr
argument to be the root window.

In Composite’s compCheckRedirect() function we now avoid calling
compAllocPixmap() and compFreePixmap() when the pWin pointer’s
parent member is NULL, as is it the case with a server’s root window.

This addresses:

https://bugs.freedesktop.org/show_bug.cgi?id=15878

composite/compwindow.c

@@ -146,7 +146,7 @@ compCheckRedirect (WindowPtr pWin)
     Bool	    should;
 
     should = pWin->realized && (pWin->drawable.class != InputOnly) &&
-	     (cw != NULL);
+	     (cw != NULL) && (pWin->parent != NULL);
 
     /* Never redirect the overlay window */
     if (cs->pOverlayWin != NULL) {