Initialize prev_payload to -1 since this packet doesnt exist

We were initializing to 0 i.e. the first packet had been received
before we've even checked for it. This meant a scenario where the first
packet i.e. one with packet ID 0 was absent would be missed from the
both send and receive packet absent list.

ansible/roles/test/files/ptftests/py3/advanced-reboot.py

@@ -1951,7 +1951,7 @@ def examine_flow(self, filename=None):
         self.fails['dut'].clear()
         prev_payload = None
         if packets:
-            prev_payload, prev_time = 0, 0
+            prev_payload, prev_time = -1, 0
             sent_payload = 0
             received_counter = 0    # Counts packets from dut.
             received_but_not_sent_packets = set()