Avoid oopsing when stress-testing open/close -- port->tty is NULL sometimes,
but is expected to be non-NULL, since dereferencing.  Receive/transmit chars
iff ASYNC_CLOSING is not set and ASYNC_INITIALIZED is set.  Thanks Sergei for
pointing this out and testing.

Cc: Sergei Organov <osv@javad.com>
Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>