Improved packet obfuscation reliability

- Fixes bugreport:7607
  http://hercules.ws/board/tracker/issue-7607-changing-emblem-with-packet-obfuscation-2-unknown-packet/
- Resolved unintended disconnections caused by invalid packets when
  obfuscation is enabled and a fragmented packet is received.
- Improved packet parsing reliability when packet obfuscation is set to
  optional.
Special thanks to serverkid for reporting the issue and Ind for figuring
out the cause and suggesting a solution.

Signed-off-by: Haru <haru@dotalux.com>