exo: Fix a nullptr dereference

In some cases (e.g. the linked bug) source can be null. SetSelection is
already a no-op for nullptr source (and invalid for nullptr data_device)
so there's no change in behaviour for the case where event_type !=
nullopt, and it fixes a nullptr dereference when event_type == nullopt.

Bug: 1138761
Test: Deploy, launch evolution, Chrome doesn't crash
Change-Id: I23da80e418834f5c1e38eecd2f0b140638b63a8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487022Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Daichi Hirono <hirono@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Fergus Dall <sidereal@google.com>
Commit-Queue: David Munro <davidmunro@google.com>
Cr-Commit-Position: refs/heads/master@{#820056}

components/exo/wayland/wl_data_device_manager.cc

@@ -306,23 +306,29 @@ class WaylandDataDeviceDelegate : public DataDeviceDelegate {
         serial_tracker_->GetEventType(serial);
     if (event_type == base::nullopt) {
       LOG(ERROR) << "The serial passed to StartDrag does not exist.";
+      if (source) {
         source->Cancelled();
+      }
       return;
     }
     if (event_type == wayland::SerialTracker::EventType::POINTER_BUTTON_DOWN &&
         serial_tracker_->GetPointerDownSerial() == serial) {
+      DCHECK(data_device);
       data_device->StartDrag(source, origin, icon,
                              ui::mojom::DragEventSource::kMouse);
     } else if (event_type == wayland::SerialTracker::EventType::TOUCH_DOWN &&
                serial_tracker_->GetTouchDownSerial() == serial) {
+      DCHECK(data_device);
       data_device->StartDrag(source, origin, icon,
                              ui::mojom::DragEventSource::kTouch);
     } else {
       LOG(ERROR) << "The serial passed to StartDrag does not match its "
                     "expected types.";
+      if (source) {
         source->Cancelled();
       }
     }
+  }
 
   void SetSelection(DataDevice* data_device,
                     DataSource* source,
@@ -331,9 +337,12 @@ class WaylandDataDeviceDelegate : public DataDeviceDelegate {
         serial_tracker_->GetEventType(serial);
     if (event_type == base::nullopt) {
       LOG(ERROR) << "The serial passed to SetSelection does not exist.";
+      if (source) {
         source->Cancelled();
+      }
       return;
     }
+    DCHECK(data_device);
     data_device->SetSelection(source);
   }