Fix compiler warning about unitialized variables.

src/backend/executor/nodeAgg.c

@@ -256,10 +256,10 @@ ExecAgg(Agg *node)
 	for(i = 0; i < nagg; i++) {
 	    AttrNumber attnum;
 	    int2 attlen;
-	    Datum newVal;
+	    Datum newVal = NULL;
 	    AggFuncInfo *aggfns = &aggFuncInfo[i];
 	    Datum args[2];
-	    Node *tagnode;
+	    Node *tagnode = NULL;
 	    
 	    switch(nodeTag(aggregates[i]->target))
 	    {