The tty_operation chars_in_buffer() is not allowed to return a negative
value to signal an error.  Corrects the problem flagged by commit
23198fda, "tty: fix chars_in_buffers".
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>