bts: Fix crash of receiving data during the release process
bts: Fix crash of receiving data during the release process

Release/Free the lapdm resources _after_ the channel has
been fully released. Do not forward data unless the lchan
is in the active state.

Reading this code again, there is probably a memory leak for
everytime the PCU will re-connect to the BTS.

 (gdb) p lchan->state
 $4 = LCHAN_S_REL_REQ
 (gdb) bt
 #0  lapd_dl_flush_hist (dl=0x40454894) at lapd_core.c:164
 #1  0x44873b54 in lapd_rx_u (lctx=0xbe9bd5a8, msg=0x92f90) at lapd_core.c:1040
 #2  lapd_ph_data_ind (msg=0x92f90, lctx=0xbe9bd5a8) at lapd_core.c:1644
 #3  0x44876d50 in l2_ph_data_ind (link_id=<optimized out>, chan_nr=<optimized out>,
     le=<optimized out>, msg=0x92f90) at lapdm.c:637
 #4  lapdm_phsap_up (oph=<optimized out>, le=<optimized out>) at lapdm.c:707
 #5  0x0000c504 in handle_ph_data_ind (l1p_msg=0x97358, data_ind=0x97420, fl1=<optimized out>)
     at l1_if.c:774
 #6  l1if_handle_ind (fl1=<optimized out>, msg=0x97358) at l1_if.c:892

geosphere/osmo-bts,geosphere/osmo-bts,osmocom/osmo-bts,geosphere/osmo-bts,telenoobie/osmo-bts,telenoobie/osmo-bts,geosphere/osmo-bts,osmocom/osmo-bts,telenoobie/osmo-bts,osmocom/osmo-bts,telenoobie/osmo-bts