libinput: Make sure we don't crash if we start without an output

src/backends/libinput/connection.cpp

@@ -477,7 +477,10 @@ void Connection::processEvents()
                 if (workspace()) {
 #ifndef KWIN_BUILD_TESTING
                     auto client = workspace()->activeClient();
-                    const auto *output = static_cast<AbstractWaylandOutput *>(client ? client->output() : tte->device()->output());
+                    auto *output = static_cast<AbstractWaylandOutput *>(client ? client->output() : tte->device()->output());
+                    if (!output) {
+                        output = static_cast<AbstractWaylandOutput *>(workspace()->activeOutput());
+                    }
                     const QPointF globalPos =
                             devicePointToGlobalPosition(tte->transformedPosition(output->modeSize()),
                                                         output);