Ensure an index that uses a whole-row Var still depends on its table.

We failed to record any dependency on the underlying table for an index
declared like "create index i on t (foo(t.*))".  This would create trouble
if the table were dropped without previously dropping the index.  To fix,
simplify some overly-cute code in index_create(), accepting the possibility
that sometimes the whole-table dependency will be redundant.  Also document
this hazard in dependency.c.  Per report from Kevin Grittner.

In passing, prevent a core dump in pg_get_indexdef() if the index's table
can't be found.  I came across this while experimenting with Kevin's
example.  Not sure it's a real issue when the catalogs aren't corrupt, but
might as well be cautious.

Back-patch to all supported versions.