libcdecl: Don't dynamically allocate null declarators.

Since identifiers now come with their own declarator allocations,
nothing actually modifies any part of an allocated null declarator.

And since null declarators are always the same (with all members zero),
we don't even need to allocate them at all.  Just use a single static
allocation and make every reference to a null declarator point to that.

Then we just need to not free null declarators in the various places
where that currently happens.