routerrpc: Check for nil IncomingCircuitKey

The grpc stream.Recv() will decode an ForwardHtlcInterceptResponse
without an IncomingCircuit, which will leave that pointer nil,
causing a nil pointer dereferece when components of in.IncomingCircuitKey
are used in the resolveFromClient() function.

This commit checks for the nil pointer and returns an error before
the rest of the parsing.

lnrpc/routerrpc/forward_interceptor.go

@@ -100,6 +100,11 @@ func (r *forwardInterceptor) onIntercept(
 func (r *forwardInterceptor) resolveFromClient(
 	in *ForwardHtlcInterceptResponse) error {
 
+	if in.IncomingCircuitKey == nil {
+		return status.Errorf(codes.InvalidArgument,
+			"CircuitKey missing from ForwardHtlcInterceptResponse")
+	}
+
 	log.Tracef("Resolving intercepted packet %v", in)
 
 	circuitKey := models.CircuitKey{