Initialize char* to NULL to remove compiler warning

This removes the following compiler warnings:

    config.c: In function ‘rewriteConfigEnumOption’:
    config.c:1352:10: warning: ‘matching_name’ may be used uninitialized in this function [-Wmaybe-uninitialized]
         line = sdscatprintf(sdsempty(),"%s %s",option,matching_name);
              ^
    config.c: In function ‘rewriteConfigSyslogfacilityOption’:
    config.c:1369:10: warning: ‘name’ may be used uninitialized in this function [-Wmaybe-uninitialized]
         line = sdscatprintf(sdsempty(),"%s %s",option,name);
              ^