Merge 10.1 into 10.2

For MDEV-15955, the fix in create_tmp_field_from_item() would cause a
compilation error. After a discussion with Alexander Barkov, the fix
was omitted and only the test case was kept.

In 10.3 and later, MDEV-15955 is fixed properly by overriding
create_tmp_field() in Item_func_user_var.

sql-common/my_time.c

@@ -184,7 +184,7 @@ static int get_date_time_separator(uint *number_of_fields, ulonglong flags,
   do
   {
     s++;
-  } while (my_isspace(&my_charset_latin1, *s));
+  } while (s < end && my_isspace(&my_charset_latin1, *s));
   *str= s;
   return 0;
 }