reiserfs: fix dereference of ERR_PTR

reiserfs_iget() returns either NULL or error code in ERR_PTR.  And we
were only checking for NULL, so in case of some other error we will try
to dereference the ERR_PTR(-errno) thinking it to be a valid pointer.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>