squid: Fix build on ARM

squid: Fix build on ARM

cc1plus: warnings being treated as errors
FtpRelay.cc: In member function 'virtual void Ftp::Relay::handleControlReply()':
FtpRelay.cc:323: error: NULL used in arithmetic
FtpRelay.cc: In member function 'void Ftp::Relay::forwardPreliminaryReply(void (Ftp::Relay::*)())':
FtpRelay.cc:362: error: NULL used in arithmetic
FtpRelay.cc: In member function 'void Ftp::Relay::proceedAfterPreliminaryReply()':
FtpRelay.cc:381: error: NULL used in arithmetic
make[4]: *** [FtpRelay.lo] Error 1