Correct and improve -Wnonnull for calls to functions with VLA arguments (PR middle...

Correct and improve -Wnonnull for calls to functions with VLA arguments (PR middle-end/97188).

Resolves:
PR middle-end/97188 - ICE passing a null VLA to a function expecting at least one element

gcc/ChangeLog:

	PR middle-end/97188
	* calls.c (maybe_warn_rdwr_sizes): Simplify warning messages.
	Correct handling of VLA argumments.

gcc/testsuite/ChangeLog:

	PR middle-end/97188
	* gcc.dg/Wstringop-overflow-23.c: Adjust text of expected warnings.
	* gcc.dg/Wnonnull-4.c: New test.