res_rtp_asterisk: Don't count 0 as a minimum lost packets

The calculated minimum lost packets represents the lowest number of
lost packets missed during an RTCP report interval. Zero of course
is the lowest, but the idea is that this value contain the lowest
number of lost packets once some have been missed.

This patch checks to make sure the number of lost packets over an
interval is not zero before checking and setting the minimum value.

Also, this patch updates the rtp lost packet test to check for
packet loss over several reports vs one.

Change-Id: I07d6e21cec61e289c2326138d6bcbcb3c3d5e008