Perl_av_extend_guts: Zero() trailing elements after unshift & resize

Perl_av_extend_guts: Zero() trailing elements after unshift & resize

Since https://github.com/Perl/perl5/commit/399fef93c903aedee05a74de780b57eeeb571b32,
trailing elements in an array that has been unshifted and resized
might not be properly Zero() initialized. This is because of faulty
arithmetic when calculating `to_null`, the number of elements to
initialize, when the array was only partially shifted.

This commit corrects the arithmetic, adds comments arount the
calculation of `to_null`, and adds a test based upon the case
provided in GH #21235.

The test added segfaults more reliably for me - almost every time -
than the originally supplied case. However, since it relies upon
uninitialized memory, it's probably still not deterministic and
somewhat dependent upon the choice of memory allocator.

Closes #21235