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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master

Details
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http://git.postgresql.org/pg/commitdiff/c5ef8ce53d37e276d70593ff0f4b06dd119cd3ff

Modified Files
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src/bin/psql/command.c |   21 ++++++++++++---------
1 file changed, 12 insertions(+), 9 deletions(-)