Check for not nil ICECandidatePair (#1253)

GetSelectedICECandidatePair can return nil for the candidate pair if not
available even if the error is not nil. Protect against the nil
de-reference panic.

pkg/rtc/transport.go

@@ -970,7 +970,7 @@ func (t *PCTransport) GetICEConnectionType() types.ICEConnectionType {
 		return unknown
 	}
 	p, err := t.getSelectedPair()
-	if err != nil {
+	if err != nil || p == nil {
 		return unknown
 	}