fix #15941 (#15948)

* fix #15941

* add testcase

* update

lib/pure/collections/sharedlist.nim

@@ -65,11 +65,14 @@ iterator items*[A](x: var SharedList[A]): A =
 proc add*[A](x: var SharedList[A]; y: A) =
   withLock(x):
     var node: SharedListNode[A]
-    if x.tail == nil or x.tail.dataLen == ElemsPerNode:
-      node = cast[type node](allocShared0(sizeof(node[])))
-      node.next = x.tail
+    if x.tail == nil:
+      node = cast[typeof node](allocShared0(sizeof(node[])))
+      x.tail = node
+      x.head = node
+    elif x.tail.dataLen == ElemsPerNode:
+      node = cast[typeof node](allocShared0(sizeof(node[])))
+      x.tail.next = node
       x.tail = node
-      if x.head == nil: x.head = node
     else:
       node = x.tail
     node.d[node.dataLen] = y

tests/stdlib/tsharedlist.nim

@@ -0,0 +1,17 @@
+import sharedlist
+
+var
+  list: SharedList[int]
+  count: int
+
+init(list)
+
+for i in 1 .. 250:
+  list.add i
+
+for i in list:
+  inc count
+
+doAssert count == 250
+
+deinitSharedList(list)