Filter.cpp: add missing locks
Try to fix potential 'Filter' namespace conflict.

Reported by @3PO in issue #60 with the following backtraces:
http://bpaste.net/raw/370021/
http://bpaste.net/raw/373927/
network mode: fix data filtering

This commit fixes the filtering when the corresponding mask of the compared
byte is eg. 0xf0 instead of 0xff (which was handled correctly).

OSCam r9743 triggers this bug by comparing only the high nibble.