Dear all,

invalid input may trigger an assert while trying to simplify an
expression involving the intrinsic UNPACK and when the constructor
is lacking sufficient valid elements.  The obvious solution is to
replace the assert by a condition that terminates simplification
in that case.

Report and testcase by Gerhard.

Regtested on x86_64-pc-linux-gnu.  OK for mainline?

This is a 10/11/12/13 regression and shall be backported.

Thanks,
Harald