Fixed bug of keys removed from tables vs 'next'

Fixed the bug that a key removed from a table might not be found
again by 'next'. (This is needed to allow keys to be removed during a
traversal.) This bug was introduced in commit 73ec04fc.

testes/nextvar.lua

@@ -359,6 +359,38 @@ end
 assert(n == 5)
 
 
+do
+  print("testing next x GC of deleted keys")
+  -- bug in 5.4.1
+  local co = coroutine.wrap(function (t)
+    for k, v in pairs(t) do
+        local k1 = next(t)    -- all previous keys were deleted
+        assert(k == k1)       -- current key is the first in the table
+        t[k] = nil
+        local expected = (type(k) == "table" and k[1] or
+                          type(k) == "function" and k() or
+                          string.sub(k, 1, 1))
+        assert(expected == v)
+        coroutine.yield(v)
+    end
+  end)
+  local t = {}
+  t[{1}] = 1    -- add several unanchored, collectable keys
+  t[{2}] = 2
+  t[string.rep("a", 50)] = "a"    -- long string
+  t[string.rep("b", 50)] = "b"
+  t[{3}] = 3
+  t[string.rep("c", 10)] = "c"    -- short string
+  t[function () return 10 end] = 10
+  local count = 7
+  while co(t) do
+    collectgarbage("collect")   -- collect dead keys
+    count = count - 1
+  end
+  assert(count == 0 and next(t) == nil)    -- traversed the whole table
+end
+
+
 local function test (a)
   assert(not pcall(table.insert, a, 2, 20));
   table.insert(a, 10); table.insert(a, 2, 20);