BUG/MEDIUM: mt_list: Make sure not to unlock the element twice

BUG/MEDIUM: mt_list: Make sure not to unlock the element twice

In mt_list_delete(), if the element was not in a list, then n and p will
point to it, and so setting n->prev and n->next will be enough to unlock it.
Don't do it twice, as once it's been done the first time, another thread may
be working with it, and may have added it to a list already, and doing it
a second time can lead to list inconsistencies.

This should be backported up to 2.8.