*new* check_check_deref.c: checking for null inconsistently

*new* check_check_deref.c: checking for null inconsistently

This looks for bugs like this:
        if (a)
                a->foo = 42;
        a->bar = 7;

This was done already in check_null_deref.c but that had lots of false
positives because of the checking that happened in macros.

This doesn't have that problem but it still has false positives because
smatch doesn't do cross function analysis or because the parent pointer
gets reassigned like this:
	if (foo->bar)
                 foo->bar->x = 42;
        foo = something();
        foo->bar->x = 99;

Signed-off-by: Dan Carpenter <error27@gmail.com>