c++: ICE with requires-expr and -Wsequence-point [PR105304]

c++: ICE with requires-expr and -Wsequence-point [PR105304]

Here we're crashing from verify_sequence_points for this requires-expr
condition because it contains a templated CAST_EXPR with empty operand,
and verify_tree doesn't ignore this empty operand only because the
manual tail recursion that it performs for unary expression trees skips
the NULL test.

	PR c++/105304

gcc/c-family/ChangeLog:

	* c-common.cc (verify_tree) [restart]: Move up to before the
	NULL test.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-requires30.C: New test.