reiserfs: possible null pointer dereference during resize

sb_read may return NULL, let's explicitly check it.  If so free new bitmap
blocks array, after this we may safely exit as it done above during bitmap
allocation.

Signed-off-by: Dmitriy Monakhov <dmonakhov@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>