MDEV-8253 EXPLAIN SELECT prints unexpected characters

Item_string::clone_item() creates a new Item_string that
points exactly to the same buffer that the original one does.
Later, Item_string::print() uses c_ptr() for the original Item_string,
which reallocs the original buffer, and the clone remain with
the old freed buffer.
Refactoring the code not to use c_ptr() in Item_string::print().

sql/sql_string.cc

@@ -954,6 +954,16 @@ void String::print(String *str) const
   str->append_for_single_quote(Ptr, str_length);
 }
 
+
+void String::print_with_conversion(String *print, CHARSET_INFO *cs) const
+{
+  StringBuffer<256> tmp(cs);
+  uint errors= 0;
+  tmp.copy(this, cs, &errors);
+  tmp.print(print);
+}
+
+
 /*
   Exchange state of this object and argument.