c: Fix ICE for invalid code with variadic and old-school prototypes [PR121507]

c: Fix ICE for invalid code with variadic and old-school prototypes [PR121507]

When mixing old-school definition without prototype and new C23
variadic functions without named argument, there can be an ICE
when trying to form the composite type.  Avoid this by letting it
fail later due to incompatible types.

	PR c/121507

gcc/c/ChangeLog:
	* c-decl.cc (start_function): Adapt condition.

gcc/testsuite/ChangeLog:
	* gcc.dg/pr121507.c: New test.