res_rtp_asterisk: More robust timestamp checking

We assume that a timestamp value of 0 represents an 'uninitialized'
timestamp, but 0 is a valid value. Add a simple wrapper to be able to
differentiate between whether the value is set or not.

This also removes the fix for ASTERISK~28812 which should not be
needed if we are checking the last timestamp appropriately.

ASTERISK-29030 #close

Change-Id: Ie70d657d580d9a1f2877e25a6ef161c5ad761cf7