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

@@ -581,7 +581,10 @@ class RTCPeerConnection {
         name: "ECDSA", namedCurve: "P-256",
       });
     }
-    this._impl.certificate = certificate;
+    // Is the PC still around after the await?
+    if (!this._closed) {
+      this._impl.certificate = certificate;
+    }
   }
 
   _resetPeerIdentityPromise() {