fix bug in `extract_esc_cap`.

fix bug in `extract_esc_cap`.

previously we were able to read past the actual end of the
input buffer (`global.in.buf`). we would not segfault due
to how bytebufs are implemented, but we could erroneously
re-process segments of input. in practice i think this
could only occur with an instantaneous burst of tty input
greater than the read buffer in `wait_event` (64 bytes). i
noticed this by accidentally mouse-wheeling very fast in
xfce4-terminal which, in my setup, sends a bunch of
up-arrow events (`\x1bOA`) all at once, in which case
termbox was pseduo-randomly emitting "O" and "A" events.