mi: don't crash on miPointerGetPosition for disabled devices

If a device is disabled, its master device is forcibly reset to NULL but
unlike a floating device it doesn't have a sprite allocated. Calling
miPointerGetPosition for a disabled device thus crashes.

Avoid this by returning 0/0 for any device without a miPointer.
This is a quick fix only, a proper fix for this issue is rather more
involved.

Closes #1782

Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1771>
(cherry picked from commit acbdd0ecdd)

mi/mipointer.c

@@ -718,8 +718,15 @@ miPointerSetPosition(DeviceIntPtr pDev, int mode, double *screenx,
 void
 miPointerGetPosition(DeviceIntPtr pDev, int *x, int *y)
 {
-    *x = MIPOINTER(pDev)->x;
-    *y = MIPOINTER(pDev)->y;
+    miPointerPtr pPointer = MIPOINTER(pDev);
+    if (pPointer) {
+        *x = pPointer->x;
+        *y = pPointer->y;
+    }
+    else {
+        *x = 0;
+        *y = 0;
+    }
 }
 
 /**