ldb: make ldb module programming less error prone

ldb: make ldb module programming less error prone

When a top level method in a module returns an error, it is supposed
to call ldb_module_done(). We ran across a case where this wasn't
done, and then found that in fact that are hundreds of similar cases
in our modules. It took Andrew and I a full day to work out that this
was the cause of a subtle segv in another part of the code.

To try to prevent this happening again, this patch changes
ldb_next_request() to catch the error by checking if a module
returning an error has called ldb_module_done(). If it hasn't then the
call is made on behalf of the module.