fixes #25240; forbids modifying a Deque changed while iterating over it (#25242)

fixes #25240

> Deque items behavior is not the same on 2.0.16 and 2.2.0

The behavior seems to be caused by the temp introduced for the parameter
`deq.len`, which prevents it from being evaluated multiple times

lib/pure/collections/deques.nim

@@ -200,8 +200,10 @@ iterator items*[T](deq: Deque[T]): lent T =
     let a = [10, 20, 30, 40, 50].toDeque
     assert toSeq(a.items) == @[10, 20, 30, 40, 50]
 
-  for c in 0 ..< deq.len:
+  let L = len(deq)
+  for c in 0 ..< L:
     yield deq.data[(deq.head + c.uint) and deq.mask]
+    assert(len(deq) == L, "the length of the Deque changed while iterating over it")
 
 iterator mitems*[T](deq: var Deque[T]): var T =
   ## Yields every element of `deq`, which can be modified.
@@ -215,8 +217,10 @@ iterator mitems*[T](deq: var Deque[T]): var T =
       x = 5 * x - 1
     assert $a == "[49, 99, 149, 199, 249]"
 
-  for c in 0 ..< deq.len:
+  let L = len(deq)
+  for c in 0 ..< L:
     yield deq.data[(deq.head + c.uint) and deq.mask]
+    assert(len(deq) == L, "the length of the Deque changed while iterating over it")
 
 iterator pairs*[T](deq: Deque[T]): tuple[key: int, val: T] =
   ## Yields every `(position, value)`-pair of `deq`.
@@ -226,8 +230,10 @@ iterator pairs*[T](deq: Deque[T]): tuple[key: int, val: T] =
     let a = [10, 20, 30].toDeque
     assert toSeq(a.pairs) == @[(0, 10), (1, 20), (2, 30)]
 
-  for c in 0 ..< deq.len:
+  let L = len(deq)
+  for c in 0 ..< L:
     yield (c, deq.data[(deq.head + c.uint) and deq.mask])
+    assert(len(deq) == L, "the length of the Deque changed while iterating over it")
 
 proc contains*[T](deq: Deque[T], item: T): bool {.inline.} =
   ## Returns true if `item` is in `deq` or false if not found.