android: prevent null pointer exception on start

Change-Id: Ifb59ba0cd634d9753f90716bda2af4f58d576afa
Reviewed-on: https://gerrit.libreoffice.org/41173
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
diff --git a/android/source/src/java/org/mozilla/gecko/gfx/JavaPanZoomController.java b/android/source/src/java/org/mozilla/gecko/gfx/JavaPanZoomController.java
index d356821..cf5cd25 100644
--- a/android/source/src/java/org/mozilla/gecko/gfx/JavaPanZoomController.java
+++ b/android/source/src/java/org/mozilla/gecko/gfx/JavaPanZoomController.java
@@ -930,7 +930,10 @@ class JavaPanZoomController

    @Override
    public boolean onDown(MotionEvent motionEvent) {
        mWaitForDoubleTap = mTarget.getZoomConstraints().getAllowDoubleTapZoom();
        if (mTarget.getZoomConstraints() != null)
            mWaitForDoubleTap = mTarget.getZoomConstraints().getAllowDoubleTapZoom();
        else
            mWaitForDoubleTap = false;
        return false;
    }