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.