sshpeer: check pipe validity before forwarding output from it

After the previous fix, fileobjectproxy._observedcall() (called when
win32.peekpipe() accesses .fileno) started exploding.  With this fix, similar
checks are needed inside debugwireproto().  Since that is hardcoded to not use
os.devnull, IDK if those are worth fixing.

mercurial/sshpeer.py

@@ -32,6 +32,7 b' def _forwardoutput(ui, pipe):'
     """display all data currently available on pipe as remote output.
 
     This is non blocking."""
+    if pipe:
     s = util.readpipe(pipe)
     if s:
         for l in s.splitlines():