Avoid null dereference in BDB dbtree error case

An error case in __bt_first would deference a null pointer.  This is
an old upstream BDB bug.  Use a separate variable to hold the result
of mpool_get() until it has been checked.  Reported by Nickolai
Zeldovich <nickolai@csail.mit.edu>.

https://github.com/krb5/krb5/commit/f5345bba2a993066f9b886dae491d211ed9be057
Author: Greg Hudson <ghudson@mit.edu>
Commit: f5345bba2a993066f9b886dae491d211ed9be057
Branch: master
 src/plugins/kdb/db2/libdb2/btree/bt_seq.c |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)