PackHexLine fails reading odd length packets

[mills95]

TcpdumpEventParser::PackHexLine only parses 16-bit tuples and a packet with an odd number of bytes causes TcpdumpEventParser::GetNextPacketEvent to get out of alignment and read the next line as if it were part of the hex line resulting in a skipped packet.

Also in TcpdumpEventParser::GetNextPacketEvent, timestamp seconds are read as floats but cast to double when generating theTime. This is not a bug, but should just read as double to prevent out-of-order packets due to floating point conversion/rounding (also better when using tcpdump --nano option).