check_memory: eliminate some false positives

check_memory:  eliminate some false positives

Originally check_memory was going to try find memory leaks.  Then checking
for double frees was tacked on.

For checking for leaks I wanted to use get_variable_from_expr_complex()
because that eliminated some false positives.  But for checking for
double frees it added false positives:
kfree(a[i]);
i++;
kfree(a[i]);  // double free false positive printed here.

It's not a very good check.  I will rewrite it eventually but for now I may
as well commit these fixes which I wrote earlier.

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