Bug fixed

null_string wasn't known in libmysqld/lib_sql.cc, that caused compiler's error
i put the declaration of the null_string in mysql_priv.h and renamed it
to my_null_string to reduce the probability of name's intersections

libmysqld/lib_sql.cc

@@ -752,7 +752,7 @@ bool setup_params_data_withlog(st_prep_stmt *stmt)
       if (*client_param->is_null)
       {
         param->maybe_null= param->null_value= 1;
-        res= &null_string;
+        res= &my_null_string;
       }
       else
       {

sql/mysql_priv.h

@@ -799,6 +799,7 @@ extern I_List<NAMED_LIST> key_caches;
 extern MY_BITMAP temp_pool;
 extern DATE_FORMAT dayord;
 extern String my_empty_string;
+extern String my_null_string;
 extern SHOW_VAR init_vars[],status_vars[], internal_vars[];
 extern struct show_table_type_st table_type_vars[];
 extern SHOW_COMP_OPTION have_isam;

sql/sql_prepare.cc

@@ -76,7 +76,7 @@ Long data handling:
 
 #define STMT_QUERY_LOG_LENGTH 8192
 
-String null_string("NULL", 4, default_charset_info);
+String my_null_string("NULL", 4, default_charset_info);
 
 /*
   Find prepared statement in thd
@@ -428,7 +428,7 @@ static bool insert_params_withlog(PREP_STMT *stmt, uchar *pos, uchar *read_pos)
       if (IS_PARAM_NULL(pos,param_no))
       {
         param->maybe_null= param->null_value= 1;
-        res= &null_string;
+        res= &my_null_string;
       }
       else
       {