pgsql: Don't call simplify_aggref with a NULL PlannerInfo

Don't call simplify_aggref with a NULL PlannerInfo

42473b3b3 added prosupport infrastructure to allow simplification of
Aggrefs during constant-folding. In some cases the context->root that's
given to eval_const_expressions_mutator() can be NULL. 42473b3b3 failed
to take that into account, which could result in a crash.

To fix, add a check and only call simplify_aggref() when the PlannerInfo
is set.

Author: David Rowley <dgrowleyml(at)gmail(dot)com>
Reported-by: Birler, Altan <altan(dot)birler(at)tum(dot)de>
Discussion: https://postgr.es/m/132d4da23b844d5ab9e352d34096eab5@tum.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/5424f4da9031ac4681ab002d20f021232070c96a

Modified Files
--------------
src/backend/optimizer/util/clauses.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)