[Windows] Panic when no cams are connected

[RivenSkaye]

Not sure if this applies to other OS options as well, but I didn't notice my USB cam was unplugged, and it gave me a nasty panic from nokhwa-bindings-windows/src/lib.rs, on line 278 due to the inner assertions in from_raw_parts.

The fix is rather easy; if count > 0 { <current L278-284 go here> } is all it takes to prevent the whole panic. And I don't think a single line fix warrants a PR