In Storable.xs fix #80074, caused by the Perl stack moving when expanded.

In Storable.xs fix #80074, caused by the Perl stack moving when expanded.

cbc736f3c4431a04 refactored Storable::{net_,}pstore to simplify the logic in
their caller, Storable::_store(). However, it introduced a bug, by assigning
the result of do_store() to a location on the Perl stack, which fails if the
Perl stack moves, because it was reallocated. Fix this assumption, and add a
test which causes the Perl stack to expand during the call to do_store().