NULL terminate device unique ID before use

bug: 12228689

If the device ID returned from OEMCrypto_GetDeviceUniqueId is
not NULL terminated in the OEM code, trailing garbage characters
may be included in the license request's client_identification
field, which could be rejected by the server's utf8 parser if
they are invalid characters, causing a license request failure.

The code for CryptoSession::GetDeviceUniqueId should use the
updated id_length from OEMCrypto_GetDeviceUniqueId to adjust
the length of the *device_id string before returning the result
to the caller.

Change-Id: I659866d4234d4f21ec051590fc7bc6367904a48a

libwvdrmengine/cdm/core/src/crypto_session.cpp

@@ -155,6 +155,9 @@ bool CryptoSession::GetDeviceUniqueId(std::string* device_id) {
     return false;
   }
 
+  id.resize(id_length + 1);
+  id[id_length] = '\0';
+
   *device_id = reinterpret_cast<const char*>(&id[0]);
   return true;
 }