VideoPlayer: fix seek at opening

The length can be negative during initialisation.

vlc-android/src/org/videolan/vlc/gui/video/VideoPlayerActivity.java

@@ -2507,7 +2507,7 @@ public class VideoPlayerActivity extends AppCompatActivity implements IVLCVout.C
     private void seek(long position, float length) {
         mForcedTime = position;
         mLastTime = mService.getTime();
-        if (length == 0f)
+        if (length <= 0f)
             mService.setTime(position);
         else
             mService.setPosition(position / length);