Make *{undef} self-consistent

Make *{undef} self-consistent

Commit afd1915d made filehandle vivification work on hash and array
elements, but in doing so it accidentally changed

    *{;undef} = 3;

to do the same thing as

    *{""} = 3;

while leaving

    *{$some_undefined_variable}

an error.

This commit adjusts the if() conditions in S_rv2gv (formerly in
pp_rv2gv) in pp.c to make PL_sv_undef follow the same path as before.

It also removes the uninit tests from lib/warnings/pp, since they
are now errors.  The uninit warning in rv2gv is only triggered now
when it is implicit, as in close().  That is already tested in
lib/warnings/9uninit.