librpc: Check for NULL pointer in value() in ntlmssp_AUTHENTICATE

This allows ndrdump --validate to avoid following a NULL pointer when re-pushing
a valid but unusual input.

It also avoids an issue if the Samba server code were to provide a response
without an EncryptedRandomSessionKey.

At this stage ntlmssp.idl is not used for this, instead the packets are
generated with msrpc_gen().

Found by Douglas Bagnall using Hongfuzz and the new fuzz_ndr_X
fuzzer.

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

Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Wed Nov 20 06:06:29 UTC 2019 on sn-devel-184

librpc/idl/ntlmssp.idl

@@ -271,7 +271,7 @@ interface ntlmssp
 		[value(ndr_ntlmssp_string_length(NegotiateFlags, Workstation))] uint16 WorkstationLen;
 		[value(WorkstationLen)] uint16 WorkstationMaxLen;
 		[relative] [subcontext(0),subcontext_size(WorkstationLen)] [flag(ndr_ntlmssp_negotiated_string_flags(r->NegotiateFlags))] string *Workstation;
-		[value(EncryptedRandomSessionKey->length)] uint16 EncryptedRandomSessionKeyLen;
+		[value(EncryptedRandomSessionKey == NULL ? 0 : EncryptedRandomSessionKey->length)] uint16 EncryptedRandomSessionKeyLen;
 		[value(EncryptedRandomSessionKeyLen)] uint16 EncryptedRandomSessionKeyMaxLen;
 		[relative] [subcontext(0),subcontext_size(EncryptedRandomSessionKeyLen)] DATA_BLOB *EncryptedRandomSessionKey;
 		NEGOTIATE NegotiateFlags;