Changes

rtp2trau_hr16: set D bits correctly

The original code failed to do the step of osmo_pbit2ubit() for
transferring HRv1 payload bits from packed to unpacked form,
instead doing the bogon of memcpy()ing the RTP payload directly
into tf->d_bits array - fix it.  Also set all D bits to zeros
in the case of empty payload input - not ideal, but better than
transmitting uninitialized memory content.

Change-Id: Icb6a21a12fe82f6637f327d45bd843686a90a458