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.

sql/sql_sort.h

@@ -545,6 +545,11 @@ public:
   {
     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);