HAWQ-1852. Fix find_udf

If IsBuildInFunction returns false, then find_udf should return true
directly, which means that the statement contains UDF. Otherwise, it
should continue to recursively determine the args of FuncExpr through
expression_tree_walker, because FuncExpr may contain recursive calls.
For example, suppose "A" is an UDF, then "select A::text" meets this
situation. The outter FuncExpr is "text" and the inner FuncExpr is "A".

src/backend/cdb/cdbdatalocality.c

@@ -6142,8 +6142,8 @@ bool udf_collector_walker(Node *node,
 	if(IsA(node,FuncExpr)){
 		if(!IsBuildInFunction(((FuncExpr *) node)->funcid)){
 			context->udf_exist = true;
+			return true;
 		}
-		return false;
 	}
 
 	return expression_tree_walker(node, udf_collector_walker,