append!(l1::MutableLinkedList, l2::MutableLinkedList) fails for empty l2

[leostenzel]

The append! function on master currently assumes l2.node.next != l2.node != l2.node.prev and fails if isempty(l2).

E.g.,

A = MutableLinkedList{Int}(1,2,3)
B = MutableLinkedList{Int}()
append!(A,B) # endless loop

This is not directly related to the more general issues #739 #886 #794 , which have been fixed on master.

The simplest solution seems to be adding a check for isempty(l2)? I can try making such PR later.
There don't seem to be any issues if isempty(l1).