channel_write: acknowledge transport errors

When draining data off the socket with _libssh2_transport_read() (which
in turn has to be done so that we can be sure to have read any possible
window-increasing packets), this code previously ignored errors which
could lead to nasty loops. Now all error codes except EAGAIN will cause
the error to be returned at once.

Bug: http://www.libssh2.org/mail/libssh2-devel-archive-2012-03/0068.shtml
Reported by: Matthew Booth

src/channel.c

@@ -2008,6 +2008,9 @@ _libssh2_channel_write(LIBSSH2_CHANNEL *channel, int stream_id,
             rc = _libssh2_transport_read(session);
         while (rc > 0);
 
+        if((rc < 0) && (rc != LIBSSH2_ERROR_EAGAIN))
+            return rc;
+
         if(channel->local.window_size <= 0)
             /* there's no room for data so we stop */
             return (rc==LIBSSH2_ERROR_EAGAIN?rc:0);