coroutines: Build pointer initializers with nullptr_node [PR107768]

coroutines: Build pointer initializers with nullptr_node [PR107768]

The PR reports that using integer_zero_node triggers a warning for
-Wzero-as-null-pointer-constant which comes from compiler-generated code so
makes no sense to the end user.

Co-Authored-By: Iain Sandoe <iain@sandoe.co.uk>
	PR c++/107768

gcc/cp/ChangeLog:

	* coroutines.cc (coro_rewrite_function_body): Initialize pointers
	from nullptr_node.  (morph_fn_to_coro): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/pr107768.C: New test.

(cherry picked from commit 0b1d66658ecdcc3d9251641a0b902b4c73ace303)