Fix errors being hidden by Protobuf warning-disabling

(This is a merge of http://go/wvgerrit/140850.)

This patch fixes a number of minor issues in the codebase (mostly
instances of 0-as-nullptr, but also some member shadowing and a missing
override) that were being hidden by the fact that depending on Protobuf
disables these diagnostics. And which will be unhidden when a later
patch removes that behavior from Protobuf.

Bug: 208304830
Test: x86-64
Change-Id: I4b0b1264748880b3726a6388d589868d898f949e

libwvdrmengine/oemcrypto/test/oec_session_util.cpp

@@ -1273,7 +1273,7 @@ Session::Session()
       forced_session_id_(false),
       session_id_(0),
       nonce_(0),
-      public_rsa_(0) {}
+      public_rsa_(nullptr) {}
 
 Session::~Session() {
   if (!forced_session_id_ && open_) close();
@@ -1470,7 +1470,7 @@ void Session::LoadOEMCert(bool verify_cert) {
       reinterpret_cast<const unsigned char*>(public_cert.data());
   long cert_size = static_cast<long>(public_cert.size());
   boringssl_ptr<PKCS7, PKCS7_free> pkcs7(
-      d2i_PKCS7(NULL, &cert_data, cert_size));
+      d2i_PKCS7(nullptr, &cert_data, cert_size));
   ASSERT_TRUE(pkcs7.NotNull()) << "Error parsing PKCS7 message";
   ASSERT_TRUE(PKCS7_type_is_signed(pkcs7.get()))
       << "Unexpected PKCS7 message type";