kernel dump: dumpsys_gen_pa_next(): Fix "no more chunks" condition detection

In the (improbable) cases where either:

• All entries in dump_map[] are used, so there is no guard entry filled with zeros.
• Some dump region has size 0.

We would respectively access dump_map[] out-of-bounds or omit further
dump regions when iterating.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation