ash: make ${v:N:M} more robust for very large M by clamping to MIN/MAX_INT

ash: make ${v:N:M} more robust for very large M by clamping to MIN/MAX_INT

Before this patch, "${v:2:0x100000001}" = "${v:2:1}",
and similarly, constructs like "${v:2:9999999999}" may give wrong result
due to int overflows.

function                                             old     new   delta
substr_atoi                                            -      43     +43

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>