Bug #31758 inet_ntoa, oct crashes server with null+filesort

Item_func_inet_ntoa and Item_func_conv inherit 'maybe_null' flag from an
argument, which is wrong.
Both can be NULL with notnull arguments, so that's fixed.

mysql-test/r/func_str.result

@@ -711,9 +711,9 @@ Warning	1265	Data truncated for column 'format(130,10)' at row 1
 show create table t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
-  `bin(130)` char(64) NOT NULL default '',
-  `oct(130)` char(64) NOT NULL default '',
-  `conv(130,16,10)` char(64) NOT NULL default '',
+  `bin(130)` char(64) default NULL,
+  `oct(130)` char(64) default NULL,
+  `conv(130,16,10)` char(64) default NULL,
   `hex(130)` char(6) NOT NULL default '',
   `char(130)` char(1) NOT NULL default '',
   `format(130,10)` char(4) NOT NULL default '',
@@ -1075,5 +1075,17 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	0	const row not found
 Warnings:
 Note	1003	select decode(test.t1.f1,'zxcv') AS `enc` from test.t1
+drop table t1;
+create table t1 (a bigint not null)engine=myisam;
+insert into t1 set a = 1024*1024*1024*4;
+delete from t1 order by (inet_ntoa(a)) desc limit 10;
+drop table t1;
+create table t1 (a char(36) not null)engine=myisam;
+insert ignore into t1 set a = ' ';
+insert ignore into t1 set a = ' ';
+select * from t1 order by (oct(a));
+a
+
+
 drop table t1;
 End of 4.1 tests

mysql-test/t/func_str.test

@@ -721,4 +721,17 @@ explain extended select encode(f1,'zxcv') as 'enc' from t1;
 explain extended select decode(f1,'zxcv') as 'enc' from t1;
 drop table t1;
 
+#
+# Bug #31758 inet_ntoa, oct, crashes server with null + filesort 
+#
+create table t1 (a bigint not null)engine=myisam;
+insert into t1 set a = 1024*1024*1024*4;
+delete from t1 order by (inet_ntoa(a)) desc limit 10;
+drop table t1;
+create table t1 (a char(36) not null)engine=myisam;
+insert ignore into t1 set a = ' ';
+insert ignore into t1 set a = ' ';
+select * from t1 order by (oct(a));
+drop table t1;
+
 --echo End of 4.1 tests

sql/item_strfunc.h

@@ -531,6 +531,7 @@ public:
   {
     collation.set(default_charset());
     decimals=0; max_length=64;
+    maybe_null= 1;
   }
 };
 
@@ -623,7 +624,7 @@ public:
     }
   String* val_str(String* str);
   const char *func_name() const { return "inet_ntoa"; }
-  void fix_length_and_dec() { decimals = 0; max_length=3*8+7; }
+  void fix_length_and_dec() { decimals = 0; max_length=3*8+7; maybe_null=1;}
 };
 
 class Item_func_quote :public Item_str_func