[perl #36347] Object destruction incomplete

[perl #36347] Object destruction incomplete

do_clean_objs only looks for objects referenced by RVs, so blessed
array references and lexical variables (and probably other SVs, too)
are not DESTROYed.

This commit adds a new visit() call to sv_clean_objs, which curses
(DESTROYs and un-blesses, leaving the reference count as it is) any
objects that are still left after do_clean_named_io_objs. The new
do_curse routine (a pointer to which is passeds to visit()) follows
do_clean_named_io_objs’ example and explicitly skips the STDOUT and
STDERR handles, in case destructors need to use them.

The cursing code, which is now called from two places, is moved out of
sv_clear and put in its own routine. The check that the reference
count is zero does not apply when called from sv_clean_objs, so the
new S_curse routine takes a boolean argument that determines whether
that check should take place.