iwlwifi: dbg: set debug descriptor to NULL outside of iwl_fw_free_dump_desc

To avoid static analysis warning and to make the flow more readable, set
the debug descriptor to NULL outside iwl_fw_free_dump_desc and only in
the required places.

Signed-off-by: Shahar S Matityahu <shahar.s.matityahu@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200424194456.5d5c50750a52.I17e33fc268c2097b7c42877f86cef2aa163b913a@changeid