Bug 38420: Xvfb crashes in miInitVisuals() when started with depth=2

https://bugs.freedesktop.org/show_bug.cgi?id=38420

Exit with fatal error message, not segfault.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Jeremy Huddleston <jeremyhu@apple.com>
Signed-off-by: Keith Packard <keithp@keithp.com>
(cherry picked from commit 7d50211ab5)

hw/vfb/InitOutput.c

@@ -864,6 +864,8 @@ vfbScreenInit(int index, ScreenPtr pScreen, int argc, char **argv)
 				   (1 << DirectColor)),
 				  10, TrueColor, 0x3ff00000, 0x000ffc00, 0x000003ff);
 	break;
+    default:
+	return FALSE;
     }
 
     miSetPixmapDepths ();