pgsql: Silence another gcc 11 warning.

Silence another gcc 11 warning.

Per buildfarm and local experimentation, bleeding-edge gcc isn't
convinced that the MemSet in reorder_function_arguments() is safe.
Shut it up by adding an explicit check that pronargs isn't negative,
and by changing MemSet to memset.  (It appears that either change is
enough to quiet the warning at -O2, but let's do both to be sure.)

Branch
------
REL9_5_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/d777e27c80f83e1177393641b99b1c7c4b19af17

Modified Files
--------------
src/backend/optimizer/util/clauses.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)