avoid KeyError in edge case of yamux handler

There are two edge cases that currently trigger `KeyError`:

1. `await newStream.open()` succeeds, but Chronos injects an implicit
   suspend point when an `{.async.}` function that suspended returns.
   During this implicit suspend point, another task could get scheduled
   and close the stream, which removes it from `m.channels` again.
   `let channel = m.channels[header.streamId` then gives `KeyError`.
   More explanation of implicit suspend point on return after await:
   status-im/nim-chronos#273

2. During the `Flush the data` block, `await m.connection.readExactly`
   is called which may suspend. If it suspends, same as above, another
   task may get scheduled that closes the stream, also resulting in the
   `KeyError` below.

The `YamuxError` is subsequently properly handled below.

libp2p/muxers/yamux/yamux.nim

@@ -525,7 +525,12 @@ method handle*(m: Yamux) {.async.} =
               await m.connection.readExactly(addr buffer[0], int(header.length))
           continue
 
-        let channel = m.channels[header.streamId]
+        let channel =
+          try:
+            m.channels[header.streamId]
+          except KeyError:
+            raise newException(YamuxError,
+              "Stream was cleaned up before handling data: " & $header.streamId)
 
         if header.msgType == WindowUpdate:
           channel.sendWindow += int(header.length)