XWin: Fix infinite loop in GetShift()

GetShift(int mask) can be called with mask==0, causing
it to go into an infinite loop.

Note: GetShift(mask) will return 0 for a mask of
both 0 and -1. The assumption is that if mask == 0,
then the corresponding bits for which we're calculating
the shift, are also 0.

hw/xwin/glx/indirect.c

@@ -1597,8 +1597,7 @@ static int
 GetShift(int mask)
 {
     int shift = 0;
-
-    while ((mask &1) == 0) {
+    while (mask > 1) {
         shift++;
         mask >>=1;
     }