The bug is a regression introduced by the patch for bug32798.

The code in Item_func_group_concat::clear() relied on the 'distinct'
variable to check if 'unique_filter' was initialized. That, however,
is not always valid because Item_func_group_concat::setup() can do
shortcuts in some cases w/o initializing 'unique_filter'.

Fixed by checking the value of 'unique_filter' instead of 'distinct'
before dereferencing.