Reset grab->next to NULL when we're creating a grab.

This is definitely necessary in CheckDeviceGrabAndHintWindow (thanks to Paulo
Zanoni for spotting it). We're resetting it everywhere else too, just to be
save.

dix/grabs.c

@@ -106,6 +106,7 @@ CreateGrab(
     grab->confineTo = confineTo;
     grab->cursor = cursor;
     grab->genericMasks = NULL;
+    grab->next = NULL;
     if (cursor)
 	cursor->refcnt++;
     return grab;