fixes #8468, stdlib.encodings.convert not crash anymore on windows (#8470)

tests/stdlib/tencoding.nim

@@ -0,0 +1,21 @@
+discard """
+  output: '''OK'''
+"""
+
+#bug #8468
+
+import encodings, strutils
+
+when defined(windows):
+  var utf16to8 = open(destEncoding = "utf-16", srcEncoding = "utf-8")
+  var s = "some string"
+  var c = utf16to8.convert(s)
+
+  var z = newStringOfCap(s.len * 2)
+  for x in s:
+    z.add x
+    z.add chr(0)
+
+  doAssert z == c
+
+echo "OK"