MDEV-22836: Server crashes in err_conv / ErrBuff::set_str

The issue here is charset for Sort_param::tmp_buffer is cleared when bzero is done for Sort_param.
Make sure to set the charset explicitly in the constructor for tmp_buffer.

mysql-test/main/order_by.result

@@ -3955,4 +3955,14 @@ ORDER BY surname_first ASC LIMIT 1 OFFSET 1;
 name	surname_first
 Charles Dickens	Dickens, Charles
 DROP TABLE t1;
+#
+# MDEV-22836: Server crashes in err_conv / ErrBuff::set_str
+#
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (0),(1);
+SELECT * FROM t1 ORDER BY CONVERT(AES_ENCRYPT(1,a), CHAR(4));
+a
+1
+0
+DROP TABLE t1;
 # End of 10.5 tests

mysql-test/main/order_by.test

@@ -2454,4 +2454,15 @@ ORDER BY surname_first ASC LIMIT 1 OFFSET 1;
 
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-22836: Server crashes in err_conv / ErrBuff::set_str
+--echo #
+
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (0),(1);
+--disable_warnings
+SELECT * FROM t1 ORDER BY CONVERT(AES_ENCRYPT(1,a), CHAR(4));
+--enable_warnings
+DROP TABLE t1;
+
 --echo # End of 10.5 tests

sql/sql_sort.h

@@ -545,6 +545,11 @@ class Sort_param {
   {
     memset(reinterpret_cast<void*>(this), 0, sizeof(*this));
     tmp_buffer.set_thread_specific();
+    /*
+      Fix memset() clearing the charset.
+      TODO: The constructor should be eventually rewritten not to use memset().
+    */
+    tmp_buffer.set_charset(&my_charset_bin);
   }
   void init_for_filesort(uint sortlen, TABLE *table,
                          ha_rows maxrows, bool sort_positions);