fixes #25173; SinglyLinkedList.remove broken / AssertionDefect (#25175)

fixes #25173

Author: ringabout

lib/pure/collections/lists.nim

@@ -712,6 +712,8 @@ proc remove*[T](L: var SinglyLinkedList[T], n: SinglyLinkedNode[T]): bool {.disc
     L.head = n.next
     if L.tail.next == n:
       L.tail.next = L.head # restore cycle
+    if L.tail == n:
+      L.tail = nil # reset tail if we removed the last node
   else:
     var prev {.cursor.} = L.head
     while prev.next != n and prev.next != nil:

tests/stdlib/tlists.nim

@@ -273,5 +273,17 @@ template main =
     list.add(n4)
     doAssert list.toSeq == @["sonic", "the", "hedgehog"]
 
+
+  block:
+    var list = initSinglyLinkedList[int]()
+
+    list.add(4)
+    list.remove(list.head)
+
+    list.add(5)
+    list.remove(list.head)
+
+    list.add(6)
+
 static: main()
 main()