The compiler warnings fixed.

Author: sanja-byelkin

sql/item_cmpfunc.cc

@@ -2692,7 +2692,7 @@ Item_func_if::str_op(String *str)
  String *res=arg->val_str(str);
  if (res)
  res->set_charset(collation.collation);
- if (null_value=arg->null_value)
+ if ((null_value=arg->null_value))
  res= NULL;
  return res;
 }
@@ -2704,7 +2704,7 @@ Item_func_if::decimal_op(my_decimal *decimal_value)
  DBUG_ASSERT(fixed == 1);
  Item *arg= args[0]->val_bool() ? args[1] : args[2];
  my_decimal *value= arg->val_decimal(decimal_value);
- if (null_value= arg->null_value)
+ if ((null_value= arg->null_value))
  value= NULL;
  return value;
 }