hmp: Guard against misuse of hmp_handle_error()

Null errp argument makes no sense.  Assert it's not null, to make this
explicit, and guard against misuse.  All current callers pass non-null
errp.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
