use python_ping as module does not calculate_packet_average

[hnkeyang]

move print_stats function calculate_packet_average code line 635、636 to the begin of of print_stats function will fix this.

[pferate]

It will only calculate the average if self.stats.packets_received > 0, otherwise it should be the default value of 0.0. The only time self.stats.average_time is used is immediately after self.calculate_packet_average() at line 636.