[519] Wimp crash on bad title bar validation

Reset the redrawhandle when creating a new window, so an empty title validation string
doesn't cause an unwanted page in of another task. This could lead to a subsequent abort
if the wrongly paged in task had a smaller slot when attempting to dereference the
pointer.

s/Wimp02

@@ -4030,6 +4030,7 @@ SWIWimp_SetExtent
 
 checkextent
         Push    "cx0,cy0,cx1,cy1,LR"
+        BL      checkredrawhandle               ; ensure no task switch for validation string reads
 ;
         BL      minwindowx
         Pull    "cx0,cy0,cx1,cy1,PC",VS