UBI: eliminate possible undefined behaviour

The assignment to pos when rb is finally NULL is undefined behaviour.
Upon seeing that assignment, GCC may assume that rb is not NULL, and
the loop condition ``rb'' may be optimised away.
Signed-off-by: NPhil Carmody <ext-phil.2.carmody@nokia.com>
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>

drivers/mtd/ubi/ubi.h

@@ -579,7 +579,8 @@ void ubi_do_get_volume_info(struct ubi_device *ubi, struct ubi_volume *vol,
 	for (rb = rb_first(root),                                            \
 	     pos = (rb ? container_of(rb, typeof(*pos), member) : NULL);     \
 	     rb;                                                             \
-	     rb = rb_next(rb), pos = container_of(rb, typeof(*pos), member))
+	     rb = rb_next(rb),                                               \
+	     pos = (rb ? container_of(rb, typeof(*pos), member) : NULL))
 
 /**
  * ubi_zalloc_vid_hdr - allocate a volume identifier header object.