Initialize offset for adaptive jitter buffer

Initialize offset for adaptive jitter buffer

When the adaptive jitter buffer is enabled in sip.conf, the first frame placed
in the jitter buffer fails with something like:

jb_warning_output: Resyncing the jb. last_delay 0, this delay -215886466,
threshold 1000, new offset 215886466

This happens because the offset is not initialized before calling jb_put(). This
patch modifies jb_put_first_adaptive() to set the offset to the frame's
timestamp.

Review: https://reviewboard.asterisk.org/r/1041/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@297310 65c4cc65-6c06-0410-ace0-fbb531ad65f3