lchan_set_state(): also free pending messages if any
lchan_set_state(): also free pending messages if any

A logical channel may have associated messages, which are pending
for transmission until some event occurs.  Do not leak them.

Change-Id: I3264e6caa007e9e9cb9c1c2f8627c408bbb28b9e

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