[perl #107000] Don’t leak if hh copying dies

[perl #107000] Don’t leak if hh copying dies

When %^H is copied on entering a new scope, if it happens to have been
tied it can die.  This was resulting in leaks, because no protections
were added to handle that case.

The two things that were leaking were the new hash in hv_copy_hints_hv
and the new value (for an element) in newSVsv.

By fixing newSVsv itself, this also fixes any potential leaks when
other pieces of code call newSVsv on explosive values.