raise an ValueError in getvalue() on closed StringIO (closes #12161)

Thanks for Catalin Iacob for the patch.
diff --git a/Lib/StringIO.py b/Lib/StringIO.py
index 340fae1..f74a066 100644
--- a/Lib/StringIO.py
+++ b/Lib/StringIO.py
@@ -266,6 +266,7 @@
         8th bit) will cause a UnicodeError to be raised when getvalue()
         is called.
         """
+        _complain_ifclosed(self.closed)
         if self.buflist:
             self.buf += ''.join(self.buflist)
             self.buflist = []
diff --git a/Lib/test/test_StringIO.py b/Lib/test/test_StringIO.py
index 4774c6d..76fc3d7 100644
--- a/Lib/test/test_StringIO.py
+++ b/Lib/test/test_StringIO.py
@@ -100,6 +100,10 @@
         self._fp.close()
         self.assertRaises(ValueError, self._fp.next)
 
+    def test_getvalue(self):
+        self._fp.close()
+        self.assertRaises(ValueError, self._fp.getvalue)
+
 class TestStringIO(TestGenericStringIO):
     MODULE = StringIO