pgsql: Fix null pointer dereference in "\c" psql command.

Fix null pointer dereference in "\c" psql command.

The psql crash happened when no current connection existed.  (The second
new check is optional given today's undocumented NULL argument handling
in PQhost() etc.)  Back-patch to 9.0 (all supported versions).

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/6087bf1a198e42b811637101580af51a55fac505

Modified Files
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src/bin/psql/command.c |    3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)