ssh2_try_send: don't try sending if a channel is half-open.

When ssh2_connection_filter_queue is _receiving_ messages about a
channel from the other end, it carefully checks if the channel
referred to is half-open. But we weren't exercising the same caution
before beginning to _send_ channel data, and we should, because in
that situation important fields like c->remwinsize aren't even
initialised yet.

This can come up, for example, due to typeahead in the main session
window before the server has sent OPEN_CONFIRMATION.

ssh2connection.c

@@ -1060,6 +1060,7 @@ static size_t ssh2_try_send(struct ssh2_channel *c)
     PktOut *pktout;
     size_t bufsize;
 
+    if (!c->halfopen) {
         while (c->remwindow > 0 &&
                (bufchain_size(&c->outbuffer) > 0 ||
                 bufchain_size(&c->errbuffer) > 0)) {
@@ -1085,6 +1086,7 @@ static size_t ssh2_try_send(struct ssh2_channel *c)
             bufchain_consume(buf, data.len);
             c->remwindow -= data.len;
         }
+    }
 
     /*
      * After having sent as much data as we can, return the amount