Change one of the conditionals to look for 'none', based on body
of comment above.

Error caught by PVS-Studio (see http://www.viva64.com/en/b/0277/)
size_t is an unsigned type, so it'll never be > -1; however, -1 is
still used by readline_shell_write (in ext/readline/readline_cli.c)
to signal an error condition, so we can use != as the comparison;
if anything ever writes size_t bytes, we have a different problem.

Error caught by PVS-Studio (see http://www.viva64.com/en/b/0277/)
The difference between two unsigned numbers would only be < 1 when
they exactly the same, so might as well check that instead of the
subtraction.

Error caught by PVS-Studio (see http://www.viva64.com/en/b/0277/)