Added function comment

sql/item.cc:
  Smple optimization

sql/field.h

@@ -92,6 +92,7 @@ class Field
 	utype unireg_check_arg, const char *field_name_arg,
 	struct st_table *table_arg);
   virtual ~Field() {}
+  /* Store functions returns 1 on overflow and -1 on fatal error */
   virtual int  store(const char *to,uint length,CHARSET_INFO *cs)=0;
   virtual int  store(double nr)=0;
   virtual int  store(longlong nr)=0;

sql/item.cc

@@ -678,9 +678,7 @@ void Item_param::set_longdata(const char *str, ulong length)
 
 int Item_param::save_in_field(Field *field, bool no_conversions)
 {
-  THD *thd= current_thd;
-
-  DBUG_ASSERT(thd->command == COM_EXECUTE);
+  DBUG_ASSERT(current_thd->command == COM_EXECUTE);
   
   if (null_value)
     return (int) set_field_to_null(field);   

sql/item.h

@@ -146,6 +146,7 @@ class Item {
     complete fix_fields() procedure.
   */
   inline void quick_fix_field() { fixed= 1; }
+  /* Function returns 1 on overflow and -1 on fatal errors */
   virtual int save_in_field(Field *field, bool no_conversions);
   virtual void save_org_in_field(Field *field)
   { (void) save_in_field(field, 1); }