s3: lib: nmblib. Clean up and harden nmb packet processing.

s3: lib: nmblib. Clean up and harden nmb packet processing.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14239
OSS-FUZZ: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=20156
OSS-FUZZ: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=20157

Credit to oss-fuzz.
No security implications.

Signed-off-by: Jeremy Allison <jra@samba.org>
Pair programmed with: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Tue Jan 21 23:33:41 UTC 2020 on sn-devel-184