pgsql: Don't try to use a constraint name as domain name

Don't try to use a constraint name as domain name

The bug that caused this to be discovered is that the code was trying to
dereference a NULL or ill-defined pointer, as reported by Michael Mueller;
but what it was doing was wrong anyway, per Heikki.

This patch is Heikki's suggested fix.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/d665162077862ef9e5886f870bf5ce909fc9342c

Modified Files
--------------
src/backend/commands/typecmds.c |    4 ++--
1 files changed, 2 insertions(+), 2 deletions(-)