Bug#58371 Assertion failed: !s.uses_buffer_owned_by(this) with format string function

Introduced by the fix for bug#44766.

Problem: it's not correct to use args[0]->str_value as a buffer,
because args[0] may need this buffer for its own purposes.

Fix: adding a new class member tmp_value to use as return value.

  @ mysql-test/r/ctype_many.result
  @ mysql-test/t/ctype_many.test
  Adding tests

  @ sql/item_strfunc.cc
  Changing code into traditional style:
  use "str" as a buffer for the argument and tmp_value for the result value.

  @ sql/item_strfunc.h
  Adding tmp_value

mysql-test/t/ctype_many.test

@@ -211,3 +211,19 @@ SELECT min(comment),count(*) FROM t1 GROUP BY ucs2_f;
 DROP TABLE t1;
 
 # End of 4.1 tests
+
+
+--echo #
+--echo # Start of 5.1 tests
+--echo #
+
+--echo #
+--echo # Bug#58371 Assertion failed: !s.uses_buffer_owned_by(this) with format string function
+--echo #
+
+SET NAMES latin1;
+DO CONVERT(CAST(SUBSTRING_INDEX(FORMAT(1,'1111'), FORMAT('','Zpq'),1) 
+                AS BINARY(0)) USING utf8);
+--echo #
+--echo # End of 5.1 tests
+--echo #

sql/item_strfunc.cc

@@ -2761,22 +2761,16 @@ String *Item_func_conv_charset::val_str(String *str)
   DBUG_ASSERT(fixed == 1);
   if (use_cached_value)
     return null_value ? 0 : &str_value;
-  /* 
-    Here we don't pass 'str' as a parameter to args[0]->val_str()
-    as 'str' may point to 'str_value' (e.g. see Item::save_in_field()),
-    which we use below to convert string. 
-    Use argument's 'str_value' instead.
-  */
-  String *arg= args[0]->val_str(&args[0]->str_value);
+  String *arg= args[0]->val_str(str);
   uint dummy_errors;
   if (!arg)
   {
     null_value=1;
     return 0;
   }
-  null_value= str_value.copy(arg->ptr(),arg->length(),arg->charset(),
+  null_value= tmp_value.copy(arg->ptr(), arg->length(), arg->charset(),
                              conv_charset, &dummy_errors);
-  return null_value ? 0 : check_well_formed_result(&str_value);
+  return null_value ? 0 : check_well_formed_result(&tmp_value);
 }
 
 void Item_func_conv_charset::fix_length_and_dec()

sql/item_strfunc.h

@@ -713,6 +713,7 @@ public:
 class Item_func_conv_charset :public Item_str_func
 {
   bool use_cached_value;
+  String tmp_value;
 public:
   bool safe;
   CHARSET_INFO *conv_charset; // keep it public