VRMLImporter: Fix memory leaks and crashes

Move Delete calls for the Current* data members to the ImportEnd call.
There's no need to wait for the destructor to get rid of them.

Set data members to NULL after Delete calls.

Add missing Delete call to fix memory leak.

Add some missing Register calls to fix double delete crashes.

Change-Id: I166f7c7acb635456ccb3fb21f2972678c803c587
VRMLImporter: Fix crash. Avoid dereferencing NULL pointer (#1624)

Bug fixed by using code similar to the "diff" attached to the bug
report. Thanks to Steve Robbins for the diff.

The two wrl files in the bug report now work, and the bridge.wrl file
from the VTK wiki example for the VRMLImporter also works. All without
reporting memory leaks.

Change-Id: Ia4ba461155237ad3edd2075c93fb31c61bb48d75
Merge topic 'vh1624' into master

ba5bb9fc VRMLImporter: Fix crash. Avoid dereferencing NULL pointer (#1624)
a9e025f3 VRMLImporter: Fix memory leaks and crashes