Fix NULL dereferencing in mrsPeerConnectionCreate

Ensure that mrsPeerConnectionCreate() returns an error code when the
internal peer connection implementation object fails to instantiate or
initialize, instead of dereferencing it.

Author: jehumb-havok

libs/Microsoft.MixedReality.WebRTC.Native/include/peer_connection.h

@@ -48,7 +48,8 @@ class PeerConnection : public webrtc::PeerConnectionObserver,
                        public webrtc::CreateSessionDescriptionObserver {
  public:
   /// Create a new PeerConnection using the given |factory|, based on the given
-  /// |config|. This serves as the constructor for PeerConnection.
+  /// |config|. This serves as the constructor for PeerConnection, and returns a
+  /// NULL pointer on failure.
   static rtc::scoped_refptr<PeerConnection> create(
       webrtc::PeerConnectionFactoryInterface& factory,
       const webrtc::PeerConnectionInterface::RTCConfiguration& config,

libs/Microsoft.MixedReality.WebRTC.Native/src/interop/interop_api.cpp

@@ -801,6 +801,9 @@ mrsPeerConnectionCreate(PeerConnectionConfiguration config,
   // Create the new peer connection
   rtc::scoped_refptr<PeerConnection> peer =
       PeerConnection::create(*factory, rtc_config, interop_handle);
+  if (!peer) {
+    return MRS_E_UNKNOWN;
+  }
   const PeerConnectionHandle handle =
       g_factory->AddPeerConnection(std::move(peer));