scheduler: Fix reading out of buffer during tx of dummy burst on PDCH TS with EGPRS enabled
scheduler: Fix reading out of buffer during tx of dummy burst on PDCH TS with EGPRS enabled

If for whatever reason (eg fn-advance too small) there's no burst
available for a PDCH TS where EGPRS is enabled, a dummy burst of size GSM_BURST_LEN
would be selected in _sched_dl_burst(), but the nbits length would still be set to
EGPRS_BURST_LEN above by func() pointer (tx_pdtch_fn()).
As a result, trx_if_send_burst() would later read EGPRS_BURST_LEN from the
dummy burst of size GSM_BURST_LEN.

The issue was found by ASan. See OS#4606 for more info.

Fixes: OS#4606
Change-Id: Iba6ccceed5c0f1db810259768678f174d39cbf8b

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