sysmo/tch.c: Clean up use of empty buffer
sysmo/tch.c: Clean up use of empty buffer

Make code easier to read and avoid reading first byte of the buffer if size is 0.

Change-Id: I5ecfc4df5a3fcad3d3ad50bf3dd3db65b694481a

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