gdi32: Fix minor read buffer overrun in CombineRgn.

dlls/gdi32/region.c

@@ -2216,6 +2216,7 @@ static BOOL REGION_SubtractO (WINEREGION *pReg, RECT *r1, RECT *r1End,
                 if (!add_rect( pReg, left, top, r1->right, bottom )) return FALSE;
 	    }
 	    r1++;
+	    if (r1 != r1End)
 	        left = r1->left;
 	}
     }