Silence "may be used uninitialized" compiler warning.

Silence "may be used uninitialized" compiler warning.

Newer gcc warns that this "actual_arg_types" variable may be used
uninitialized, but visual inspection indicates there's no bug.  To
silence the warning, initialize the variable to zeros.

Bug: #19485
Reported-by: Hans Buschmann <buschmann@nidsa.net>
Tested-by: Erik Rijkers <er@xs4all.nl>
Tested-by: Hans Buschmann <buschmann@nidsa.net>
Reviewed-by: Tristan Partin <tristan@partin.io>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/19485-2b03231a775756f1%40postgresql.org
Discussion: https://postgr.es/m/6c52a1a6612948519468d46cb224a8c4%40nidsa.net