ash: fix handling of negative start value in ${v:start:len}

function                                             old     new   delta
subevalvar                                          1140    1168     +28

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
ash,hush: do not segfault on $((2**63 / -1))

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