ash: fail if 'shift' operand is out of range

ash: fail if 'shift' operand is out of range

If the numeric argument passed to ash's 'shift' built-in is greater than
'$#' the command performs no operation and exits successfully. It should
return a non-zero exit code instead:

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#shift

This is consistent with bash and hush.

function                                             old     new   delta
shiftcmd                                             122     120      -2

Signed-off-by: Ingo van Lil <inguin@gmx.de>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>