When fuzzing hll/pp inputs, a policy module where the value of
scope->decl_ids_len has been modified to zero makes the program abort
(when it has been compiled without -DNDEBUG).
Change the behavior to report an error message instead. This eases
fuzzing functions like policydb_read().
Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>