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_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/5f173a9f2a9726c3be9f47eb2081ec4b089bf046

Modified Files
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src/bin/psql/command.c |    6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)