c++: Don't crash upon invalid placement new operator [PR117101]

c++: Don't crash upon invalid placement new operator [PR117101]

We currently crash upon the following invalid code (notice the "void
void**" parameter)

=== cut here ===
using size_t = decltype(sizeof(int));
void *operator new(size_t, void void **p) noexcept { return p; }
int x;
void f() {
    int y;
    new (&y) int(x);
}
=== cut here ===

The problem is that in this case, we end up with a NULL_TREE parameter
list for the new operator because of the error, and (1) coerce_new_type
wrongly complains about the first parameter type not being size_t,
(2) std_placement_new_fn_p blindly accesses the parameter list, hence a
crash.

This patch does NOT address #1 since we can't easily distinguish between
a new operator declaration without parameters from one with erroneous
parameters (and it's not worth the risk to refactor and break things for
an error recovery issue) hence a dg-bogus in new52.C, but it does
address #2 and the ICE by simply checking the first parameter against
NULL_TREE.

It also adds a new testcase checking that we complain about new
operators with no or invalid first parameters, since we did not have
any.

	PR c++/117101

gcc/cp/ChangeLog:

	* init.cc (std_placement_new_fn_p): Check first_arg against
	NULL_TREE.

gcc/testsuite/ChangeLog:

	* g++.dg/init/new52.C: New test.
	* g++.dg/init/new53.C: New test.