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>

shell/ash.c

@@ -10858,7 +10858,7 @@ shiftcmd(int argc UNUSED_PARAM, char **argv)
 	if (argv[1])
 		n = number(argv[1]);
 	if (n > shellparam.nparam)
-		n = 0; /* bash compat, was = shellparam.nparam; */
+		return 1;
 	INT_OFF;
 	shellparam.nparam -= n;
 	for (ap1 = shellparam.p; --n >= 0; ap1++) {