GLcore: make googleearth not crash the server on sw-rendering.

I don't think this is the 100% correct answer as I get log spam saying
(EE) DoSwapInterval: cx = 0x98b8998, GLX screen = 0x96dd780
(EE) AIGLX: cx->pGlxScreen->swapInterval == NULL

but thats better than X exiting in my book.

GL/glx/glxglcore.c

@@ -432,6 +432,7 @@ __glXMesaScreenProbe(ScreenPtr pScreen)
     screen->base.destroy        = __glXMesaScreenDestroy;
     screen->base.createContext  = __glXMesaScreenCreateContext;
     screen->base.createDrawable = __glXMesaScreenCreateDrawable;
+    screen->base.swapInterval  = NULL;
     screen->base.pScreen       = pScreen;
 
     return &screen->base;