autoboot: fix illegal memory access when stop key and delay key are empty

If both stop key and delay key are empty, the length of these
keys is 0. The subtraction operation will cause the u_int type
variable to overflow, will cause illegal memory access in key
input loop.

This commit fixes this bug by using int type instead of u_init.
Acked-by: Heinrich Schuchardt <xypron.glpk@gmx.de>