altos/lisp: Eliminate compiler warning about array bounds at -O3

altos/lisp: Eliminate compiler warning about array bounds at -O3

Using ao_lisp_pool - 4 caused the compiler to whinge about computing
an address outside the bounds of the array. Sigh. Restructure the code
to do the adjustment-by-4 in the integer computations instead of the
pointer ones.

Signed-off-by: Keith Packard <keithp@keithp.com>