c: Stray inform note with -Waddress [PR106947]

A trivial fix for maybe_warn_for_null_address where we print an
inform note without first checking the return value of a warning
call.

	PR c/106947

gcc/c/ChangeLog:

	* c-typeck.cc (maybe_warn_for_null_address): Don't emit stray
	notes.

gcc/testsuite/ChangeLog:

	* c-c++-common/Waddress-7.c: New test.