Dear all,

I had thought that we had fixed this in the past (see PR31001),
but it did fail for me with all gcc versions I have tried (7-12)
for a slightly more elaborate case as in the old testcase.

The loop in pack_internal did try to access the first element of
the array argument to PACK even if one (or more) extents were zero.
This is not good.

Solution: check the extents and return early.  (We already do a
related check for the vector argument if present).

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

As this segfaults on valid code at runtime: I am considering
backporting this, if there are no objections.

Thanks,
Harald