Webcam should prevent double close

[edaniels]

In https://github.com/blackjack/webcam/blob/master/webcam.go#L276, a user may accidentally (due to a bug) call Close more than once. This is particularly dangerous since we're holding to a raw file descriptor number. This could play out as:

1. First close (normal, expected)
2. Some other code opens a file and gets that now unused file descriptor number
3. Second close (bug, unexpected) and now the other code has its file descriptor unexpectedly modified