pgsql: Be more paranoid about null return values from libpq status func

Be more paranoid about null return values from libpq status functions.

PQhost() can return NULL in non-error situations, namely when a Unix-socket
connection has been selected by default. That behavior is a tad debatable
perhaps, but for the moment we should make sure that psql copes with it.
Unfortunately, do_connect() failed to: it could pass a NULL pointer to
strcmp(), resulting in crashes on most platforms. This was reported as a
security issue by ChenQin of Topsec Security Team, but the consensus of
the security list is that it's just a garden-variety bug with no security
implications.

For paranoia's sake, I made the keep_password test not trust PQuser or
PQport either, even though I believe those will never return NULL given
a valid PGconn.

Back-patch to all supported branches.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/6430a11fa9bc2e56d468283ac73c9c15253ae32e

Modified Files
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src/bin/psql/command.c | 23 +++++++++++++----------
1 file changed, 13 insertions(+), 10 deletions(-)