Stop SEGV on 'our sub { syntax error }'

Fix for RT #116981. If a sub is declared with our, the name is added
to the stash early, and left with a NULL ptr if there's a syntax error
while compiling it.

Since the only time it becomes an issue is when that same name
is used in the same scope after a syntax error, what happens in
the pad is not particularly important.  The simple fix is to
simply fall back to treating it like a bareword, and pretending
it was never added to the pad in the first place.