pgsql: Fix a couple of places where psql might fail to report a suitable

Log Message:
-----------
Fix a couple of places where psql might fail to report a suitable error
if PQexec returns NULL.  These don't seem significant enough to be worth
back-patching, but they ought to get fixed ...

Modified Files:
--------------
    pgsql/src/bin/psql:
        common.c (r1.139 -> r1.140)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/common.c?r1=1.139&r2=1.140)
        tab-complete.c (r1.170 -> r1.171)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/tab-complete.c?r1=1.170&r2=1.171)