osmo-bts-trx/trx_if.c: fix: always initialize bi->burst_len for NOPE.ind
osmo-bts-trx/trx_if.c: fix: always initialize bi->burst_len for NOPE.ind

A NOPE.ind indicates absence of an Uplink burst, thus it does not
carry a burst. Let's init the burst length to avoid uninitialized
memory access in the scheduler code.

Change-Id: I77f686bf7df385215892e71733a28ff0d90d7222
Fixes: CID#205857

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