Bugfix: Avoid crash after un-plugging a webcam and then trying to re-scan the devices.

Set ptr to NULL,
so static void icvCloseCAM_V4L( CvCaptureCAM_V4L* capture ) can be called repeatedly.

This fixes a crash: After un-plugging a webcam and trying to re-scan the cameras there would be a crash due to icvCloseCAM_V4L() trying to free(capture->deviceName).