dix: Assert noPanoramiXExtension is false in PanoramiX code

When compiling with link time optimization, GCC thinks it's discovered
undefined behavior:

events.c: In function 'XineramaConfineCursorToWindow':
events.c:609:13: warning: iteration 2147483647 invokes undefined behavior [-Waggressive-loop-optimizations]
events.c:609:11: note: within this loop
events.c:605:49: warning: array subscript -1 is below array bounds of 'struct _Window *[16]' [-Warray-bounds]
events.c:606:31: warning: array subscript -1 is below array bounds of 'struct _Screen *[16]' [-Warray-bounds]
events.c:610:39: warning: array subscript -2 is below array bounds of 'struct _Screen *[16]' [-Warray-bounds]
events.c:617:38: warning: array subscript -2 is below array bounds of 'struct _Window *[16]' [-Warray-bounds]
events.c:619:35: warning: array subscript -2 is below array bounds of 'struct _Screen *[16]' [-Warray-bounds]

This results from

    i = PanoramiXNumScreens - 1;

    RegionCopy(&pSprite->Reg1, &pSprite->windows[i]->borderSize);
    off_x = screenInfo.screens[i]->x;
    off_y = screenInfo.screens[i]->y;

where GCC believes that PanoramiXNumScreens might be 0. Unfortunately
GCC is just smart enough to be an annoyance because this case is not
actually possible: XineramaConfineCursorToWindow() is only called when
noPanoramiXExtension is false, and if noPanoramiXExtension is false then
PanoramiXNumScreens must be >1 (see PanoramiXExtensionInit()).

So, add an assert(!noPanoramiXExtension), which to my surprise provides
GCC with information even in release builds and lets GCC understand that
the code is not doing anything that is undefined behavior.

I chose this solution instead of the proposed assert(i >= 0) because the
same pattern occurs in CheckVirtualMotion() but is inside an
'if (!noPanoramiXExtension)' and does not generate any warnings.

Fixes: xorg/xserver#590
Signed-off-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit 61aa40ae)