iwlwifi: plug memory modified after free

In certain situations we saw a memory modified after free. This was
tracked down to a pointer not NULLed after free and used in a different
code path. It is unclear how the race happens pending further
investigation but setting the pointer to NULL after free and adding a
check in the 2nd code path handling the case gracefully helps for now.

While here improve another debug messge in sta handling.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days