tsc can be NULL (mac80211 currently always passes NULL),
resulting in NULL-dereference. check before copying it.

Cc: stable@vger.kernel.org
Signed-off-by: NEliad Peller <eliadx.peller@intel.com>
Signed-off-by: NEmmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>