Fix btree byte swapping for overflow data

When operating on a btree database file of the opposite endianness,
libdb2 was swapping the wrong bytes if a record had a short key but
overflow data.  Fix this bug by not incrementing p when swapping a
P_BIGKEY overflow pointer, and by always skipping the full key size
before swapping a P_BIGDATA overflow pointer (instead of assuming that
a P_BIGKEY pointer always precedes a P_BIGDATA pointer).

(cherry picked from commit b6238a61769ffbf8a8ac653baeed1c3373b961f8)

https://github.com/krb5/krb5/commit/927825cdb37f5055f5c4f3acdb7969c0ac0d0047
Author: Tom Yu <tlyu@mit.edu>
Commit: 927825cdb37f5055f5c4f3acdb7969c0ac0d0047
Branch: krb5-1.14
 src/plugins/kdb/db2/libdb2/btree/bt_conv.c |   15 +++++++++------
 1 files changed, 9 insertions(+), 6 deletions(-)