Bug 1503023 - After waiting for certs make sure the PeerConnection still exists r=jib

Ensure that a PeerConnection is still open before setting the certificates.

Differential Revision: https://phabricator.services.mozilla.com/D10122

--HG--
extra : moz-landing-system : lando

dom/media/PeerConnection.js

@@ -581,8 +581,11 @@ class RTCPeerConnection {
         name: "ECDSA", namedCurve: "P-256",
       });
     }
+    // Is the PC still around after the await?
+    if (!this._closed) {
       this._impl.certificate = certificate;
     }
+  }
 
   _resetPeerIdentityPromise() {
     this._peerIdentity = new this._win.Promise((resolve, reject) => {