Fix shared_ptr usage

The std::shared_ptr type was used instead of std::tr1::shared_ptr. A NULL
pointer was also implicitly cast into a Backend pointer which caused
compilation problems on some platforms with the tr1 version of shared_ptr.

server/modules/routing/schemarouter/schemarouter.hh

@@ -35,7 +35,7 @@
 using std::list;
 using std::set;
 using std::string;
-using std::shared_ptr;
+using std::tr1::shared_ptr;
 
 using maxscale::Buffer;
 

server/modules/routing/schemarouter/schemaroutersession.cc

@@ -865,7 +865,7 @@ SBackend SchemaRouterSession::get_bref_from_dcb(DCB* dcb)
 
     // This should not happen
     ss_dassert(false);
-    return SBackend(NULL);
+    return SBackend(reinterpret_cast<Backend*>(NULL));
 }
 
 /**