Avoid shutdown on closed socket.

Fix python-websockets#1072.

src/websockets/legacy/protocol.py

@@ -1303,7 +1303,7 @@ async def close_connection(self) -> None:
                     self.logger.debug("! timed out waiting for TCP close")
 
             # Half-close the TCP connection if possible (when there's no TLS).
-            if self.transport.can_write_eof():
+            if self.transport.can_write_eof() and not self.transport.is_closing():
                 if self.debug:
                     self.logger.debug("x half-closing TCP connection")
                 self.transport.write_eof()

tests/legacy/test_protocol.py

@@ -59,6 +59,9 @@ def setup_mock(self, loop, protocol):
     def can_write_eof(self):
         return True
 
+    def is_closing(self):
+        return False
+
     def write_eof(self):
         # When the protocol half-closes the TCP connection, it expects the
         # other end to close it. Simulate that.