misc fixes

* add null checks before resuming a download
* (MissionAdapter.java) reset percent after resuming a download. prevents the "Error" string get stuck, until the download start

app/src/main/java/us/shandian/giga/ui/adapter/MissionAdapter.java

@@ -447,6 +447,7 @@ public class MissionAdapter extends Adapter<ViewHolder> {
         if (mission != null) {
             switch (id) {
                 case R.id.start:
+                    h.status.setText(UNDEFINED_SPEED);
                     h.state = -1;
                     h.size.setText(Utility.formatBytes(mission.getLength()));
                     mDownloadManager.resumeMission(mission);