Make get_cv cope with subrefs

Make get_cv cope with subrefs

When called with GV_NOADD_NOINIT, get_cv will fail an assertion if the
thingy in the stash is not a GV.

While calling with GV_NOADD_NOINIT is a strange thing to do, neverthe-
less Cpanel::JSON::XS does it with "Encode::decode", which is not an
unreasonable thing to do if it is known that the sub already exists.

This commit makes get_cv take sub refs into account, so that, when we
reënable the optimisation reverted by 9bceb75b8d9, Cpanel::JSON::XS
will continue to work with bleadperl.

(Currently, the optimisation only applies to the main package, which
is why I am able to test this now.)