Fix compilation errors/warning with gcc-5.4.0 (#137)

Warning are like this:
 error: ‘delimiter’ may be used uninitialized in this function [-Werror=maybe-uninitialized]

include/maxscale/filter.hh

@@ -215,7 +215,7 @@ public:
     static MXS_FILTER_SESSION* newSession(MXS_FILTER* pInstance, MXS_SESSION* pSession)
     {
         FilterType* pFilter = static_cast<FilterType*>(pInstance);
-        FilterSessionType* pFilterSession;
+        FilterSessionType* pFilterSession = NULL;
 
         MXS_EXCEPTION_GUARD(pFilterSession = pFilter->newSession(pSession));