Fix temporary call messages being handled without call

In cases where a rogue client, or just a simple bug, sends a temporary
call message, like CallNegotiate the messages should just be discarded
if there's no actual p2p connection created.

src/webrtc/call.ts

@@ -1286,7 +1286,7 @@ export class MatrixCall extends EventEmitter {
         // https://developer.mozilla.org/en-US/docs/Web/API/WebRTC_API/Perfect_negotiation
         const offerCollision = (
             (description.type === 'offer') &&
-            (this.makingOffer || this.peerConn.signalingState != 'stable')
+            (this.makingOffer || this.peerConn.signalingState !== 'stable')
         );
 
         this.ignoreOffer = !polite && offerCollision;
@@ -1935,6 +1935,10 @@ export class MatrixCall extends EventEmitter {
             }
         }
     }
+
+    get hasPeerConnection() {
+        return Boolean(this.peerConn);
+    }
 }
 
 async function getScreensharingStream(