l1sap: do not call msgb_l2hlen without checking
l1sap: do not call msgb_l2hlen without checking

We request the length using msgb_l2len() in two locations where whe
cannot be sure that l2h is populated. Lets check this first.

Change-Id: Ie13d0724f183ff240714dcdbd24e5a21b4276bfe
Related: OS#5645

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts