Workaround for invalid timestamps that might occur in capture files.

Encountered a problem with some capture files (generated by Wireshark): float(self.sniff_timestamp) would throw an exception on timestamp values such as "1386694288.-00019000". Apparently these values do occur in capture files; Wireshark gives a warning about the timestamp being malformed, but keeps processing the packet anyway. My workaround discards the negative value after the decimal point. Google for "wireshark fractional second" for more occurences.