Fix metadata corruption
Fix metadata corruption

Use separate pointer for payload to make sure mblk->b_rptr is not
advanced (which breaks the use of rtp_get_* macros that assume
mblk->b_rptr is pointing to the RTP header, not the payload).

Committed by: Max <msuraev@sysmocom.de>

Change-Id: I205630e1da98693ba0ac25e86e21780284185077
Related: OS#1802

osmocom/libosmo-abis,osmocom/libosmo-abis,osmocom/libosmo-abis