check_deref: ignore pointers if we assign their address to another

check_deref: ignore pointers if we assign their address to another

It's too hard to track pointers where you assign their address like this:

	char *p = NULL;
	char **str_p = &p;

I guess there were false positives where we warned that "p" was NULL after
*str_p had been updated.  This could happen in a different thread for
example and Smatch wouldn't know.

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