pgsql: Ensure that PLPGSQL_DTYPE_ROW variables have valid refnamefield

Ensure that PLPGSQL_DTYPE_ROW variables have valid refname fields.

Without this, the syntax-tree-dumping functions in pl_funcs.c crash,
and there are other places that might be at risk too.  Per report
from Pavel Stehule.

Looks like I broke this in commit f9263006d, so back-patch to v11.

Discussion: https://postgr.es/m/CAFj8pRA+3f5n4642q2g8BXCKjbTd7yU9JMYAgDyHgozk6cQ-VA@mail.gmail.com

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/6e526b78706a99dc52182387d0e6f84d05bd597a

Modified Files
--------------
src/pl/plpgsql/src/pl_comp.c | 2 ++
src/pl/plpgsql/src/pl_exec.c | 1 +
src/pl/plpgsql/src/pl_gram.y | 3 +++
src/pl/plpgsql/src/plpgsql.h | 7 ++++++-
4 files changed, 12 insertions(+), 1 deletion(-)