Fix btree byte swapping for overflow data

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).

ticket: 8485 (new)
target_version: 1.14-next
target_version: 1.13-next
tags: pullup