bpo-30980: Fix double close in asyncore.file_wrapper (#2789)

* bpo-30980: Fix close test to fail

test_close_twice was not considering the fact that file_wrapper is
duping the file descriptor. Closing the original descriptor left the
duped one open, hiding the fact that close protection is not effective.

* bpo-30980: Fix double close protection

Invalidated self.fd before closing, handling correctly the case when
os.close raises.

* bpo-30980: Fix fd leak introduced in the fixed test

Lib/test/test_asyncore.py

@@ -433,7 +433,10 @@ class FileWrapperTest(unittest.TestCase):
         f = asyncore.file_wrapper(fd)
         os.close(fd)
 
-        f.close()
+        os.close(f.fd)  # file_wrapper dupped fd
+        with self.assertRaises(OSError):
+            f.close()
+
         self.assertEqual(f.fd, -1)
         # calling close twice should not fail
         f.close()