iwlwifi: mvm: Fix possible NULL pointer dereference

iwl_mvm_te_clear_data() is called for cleanup in case sending
the HOT_SPOT_CMD failed. However, in case sending the command
caused a fw error and restart (e.g. if the command is not supported)
then the te_data pointer may no longer be valid, which leads to
a NULL pointer dereference.

Fix it by checking that the te_data pointer is not NULL before
dereferencing it.

Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>