MB-7656 Return NULL on iobuffer instantiation failure

Callers such as tree_file_open() check for failure of
couch_get_buffered_file_ops() by checking if its return
value is NULL. So on error return NULL, otherwise caller
thinks that everything succeeded.

Change-Id: If11b997e89c793cde80783a0d58695a05d7cfdca
Reviewed-on: http://review.couchbase.org/28845
Reviewed-by: Aaron Miller <apage43@ninjawhale.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Aaron Miller <apage43@ninjawhale.com>