Make get_raw_samples() use provided mem instead of allocating

Until now, Segment::get_raw_samples() was allocating the required
amount of memory and returned it to the caller to use. This way,
there was always enough memory allocated for the data that was
written to that memory location.

However, in SignalBase::conversion_thread_proc() we want to use
one memory location multiple times because we will create several
layers of libsigrok wrapper objects around it. This only works
if Segment::get_raw_samples() uses a given memory location instead
of allocating it by itself.