pgsql: Work around NetBSD shell issue in pg_upgrade test script.

Work around NetBSD shell issue in pg_upgrade test script.

The NetBSD shell apparently returns non-zero from an unset command if
the variable is already unset. This matters when, as in pg_upgrade's
test.sh, we are working under 'set -e'. To protect against this, we
first set the PG variables to an empty string before unsetting them
completely.

Error found on buildfarm member coypu, solution from Rémi Zara.

Branch
------
REL9_3_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/bd04dfba9f7e10274a6b2558e57e274403161373

Modified Files
--------------
contrib/pg_upgrade/test.sh |   20 ++++++++++++--------
1 file changed, 12 insertions(+), 8 deletions(-)