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

( New pull request for my commit that was previously made to master branch,
  see comment in https://github.com/Itseez/opencv/pull/750 )

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).