Bug#20768717: DEBUG BUILD FAILS WHEN USING GCC 5 DUE TO COMPILER WARNING

Add extra set of parentheses to remove a GCC 5 compiler warning.

Author: jhauglid

sql/gcalc_tools.cc

@@ -1,4 +1,4 @@
-/* Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
+/* Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -223,7 +223,7 @@ int Gcalc_function::count_internal()
         result= result & !next_res;
         break;
       case op_backdifference:
-        result= !result & next_res;
+        result= (!result) & next_res;
         break;
       default:
         DBUG_ASSERT(FALSE);