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 f9263006, so back-patch to v11.

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