tools: kwboot: Fix quitting terminal

tools: kwboot: Fix quitting terminal

Sometimes kwboot after quitting terminal prints error message:

  terminal: Bad address

This is caused by trying to call write() syscall with count of (size_t)-1
bytes.

When quit sequence is split into more read() calls then number of input
bytes (nin) at the end of cycle can underflow and be negative. Fix it.

Fixes: c0b6d92933a8 ("tools: kwboot: Fix detection of quit esc sequence")
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>