fix build with older gcc
    
    Older gcc (4.1.2-55.el5, 4.2.1 on FreeBSD) reports bogus warnings:
    ../../src/conf/nwfilter_conf.c:2111: warning: 'protocol' may be used
    uninitialized in this function
    ../../src/conf/nwfilter_conf.c:2110: warning: 'dataProtocolID' may be
    used uninitialized in this function
    
    Initialize them to NULL to make the compiler happy.