analyzer: only use CWE-690 for unchecked return value [PR97893]

CWE-690 is only for dereferencing an unchecked return value; for
other kinds of NULL dereference, use the parent classification, CWE-476.

gcc/analyzer/ChangeLog:
	PR analyzer/97893
	* sm-malloc.cc (null_deref::emit): Use CWE-476 rather than
	CWE-690, as this isn't due to an unchecked return value.
	(null_arg::emit): Likewise.

gcc/testsuite/ChangeLog:
	PR analyzer/97893
	* gcc.dg/analyzer/malloc-1.c: Add CWE-690 and CWE-476 codes to
	expected output.