Fix pg_get_constraintdef to cope with NOT VALID constraints

Fix pg_get_constraintdef to cope with NOT VALID constraints

This case was missed when NOT VALID constraints were first introduced in
commit 722bf7017bbe796decc79c1fde03e7a83dae9ada by Simon Riggs on
2011-02-08.  Among other things, it causes pg_dump to omit the NOT VALID
flag when dumping such constraints, which may cause them to fail to
load afterwards, if they contained values failing the constraint.

Per report from Thom Brown.