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).

(cherry picked from commit b6238a61769ffbf8a8ac653baeed1c3373b961f8)

ticket: 8485
version_fixed: 1.13.7