Preserve lexicals across dounwind() calls on perl 5.19.4 and above

unwind() used to rely on the stack being preserved when dounwind() is
called. However, starting with change 25375124, dounwind() can now call
leave_scope() which will clean up lexicals in the unwinded scopes. We
can work around this by temporarily bumping the refcount of possibly
problematic scalars before calling dounwind().

See also https://rt.perl.org/rt3/Ticket/Display.html?id=119681.