Expand SSHForwarder fix to cover both write() and write_eof()

This commit expands on commit 3d31c4c, fixing an assertion error in
both write() and write_eof() when cleaning up SSHForwarder sessions.
The previous fix only covered write_eof(). Thanks go to Mike Barry
for reporting the remaining issue and helping to test the new fix.

Author: ronf

asyncssh/forward.py

@@ -85,8 +85,13 @@ def set_peer(self, peer: 'SSHForwarder') -> None:
     def write(self, data: bytes) -> None:
         """Write data to the transport"""
 
-        assert self._transport is not None
-        self._transport.write(data)
+        if not self._transport:
+            return # pragma: no cover
+
+        try:
+            self._transport.write(data)
+        except OSError: # pragma: no cover
+            pass
 
     def write_eof(self) -> None:
         """Write end of file to the transport"""