regfio: Improve handling of malformed registry hive files

regfio: Improve handling of malformed registry hive files

* next_record: A malformed file can lead to an endless loop.
* regfio_rootkey: Supplying a malformed registry hive file to the
  registry hive I/O code can lead to out-of-bounds reads.

Test cases are included. Both issues resolved have been identified using
AddressSanitizer.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13840

Signed-off-by: Michael Hanselmann <public@hansmi.ch>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>