trx: Don't call osmo_fr_check_sid with negative 'rc'
trx: Don't call osmo_fr_check_sid with negative 'rc'

In rx_tchf_fn(), if gsm0503_tch_fr_decode() returns a negative
result, we cannot use that result as length argument to
osmo_fr_check_sid()

Change-Id: Ic4080b5bf6c865be3333f923f19a2340e1e272c8
Fixes: Coverity CID#178659

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