fix nim-lang#13496 handle tombstones

lib/pure/collections/tables.nim

@@ -1118,7 +1118,7 @@ iterator pairs*[A, B](t: TableRef[A, B]): (A, B) =
   ##   # value: [1, 5, 7, 9]
   let L = len(t)
   for h in 0 .. high(t.data):
-    if isFilled(t.data[h].hcode):
+    if isFilledAndValid(t.data[h].hcode):
       yield (t.data[h].key, t.data[h].val)
       assert(len(t) == L, "the length of the table changed while iterating over it")
 
@@ -1140,7 +1140,7 @@ iterator mpairs*[A, B](t: TableRef[A, B]): (A, var B) =
 
   let L = len(t)
   for h in 0 .. high(t.data):
-    if isFilled(t.data[h].hcode):
+    if isFilledAndValid(t.data[h].hcode):
       yield (t.data[h].key, t.data[h].val)
       assert(len(t) == L, "the length of the table changed while iterating over it")
 
@@ -1182,7 +1182,7 @@ iterator values*[A, B](t: TableRef[A, B]): B =
 
   let L = len(t)
   for h in 0 .. high(t.data):
-    if isFilled(t.data[h].hcode):
+    if isFilledAndValid(t.data[h].hcode):
       yield t.data[h].val
       assert(len(t) == L, "the length of the table changed while iterating over it")
 
@@ -1203,7 +1203,7 @@ iterator mvalues*[A, B](t: TableRef[A, B]): var B =
 
   let L = len(t)
   for h in 0 .. high(t.data):
-    if isFilled(t.data[h].hcode):
+    if isFilledAndValid(t.data[h].hcode):
       yield t.data[h].val
       assert(len(t) == L, "the length of the table changed while iterating over it")