hfsplus: prevent crash on exit from failed search

hfs_find_exit() expects fd->bnode to be NULL after a search has failed.
hfs_brec_insert() may instead set it to an error-valued pointer.  Fix
this to prevent a crash.

Link: http://lkml.kernel.org/r/803590a35221fbf411b2c141419aea3233a6e990.1530294813.git.ernesto.mnd.fernandez@gmail.com
Signed-off-by: Ernesto A. Fernandez <ernesto.mnd.fernandez@gmail.com>
Reported-by: Anatoly Trosinenko <anatoly.trosinenko@gmail.com>
Reviewed-by: Vyacheslav Dubeyko <slava@dubeyko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>