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
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/9060cb96805e0d3cfe74c0919feb5e2ce1034ffc

Modified Files
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contrib/pg_upgrade/test.sh |   22 ++++++++++++++--------
1 file changed, 14 insertions(+), 8 deletions(-)