deref_check: silence some false positives

deref_check: silence some false positives

The issue here was we had code like:

int some_function(int *p, ...)
{
	if (p)
		*p = 42;

The problem is that all the callers pass a NULL "p" pointer so instead of
saying that "p" is some non-NULL pointer, smatch instead says that it
doesn't know what "p" is, it could be anything.

So then when we check the "if (implied_not_equal(expr, 0))" setting then
it doesn't return true.  Leading to false positives later.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>