Fix getPixel for pixels with zero red value.

Remove an erroneous if that causes `getPixel()` to fail if the red value of the pixel is zero.

I'm not sure what the original intent of the if check was, and I have made no attempt to preserve that intent. It is possible that it wanted to avoid out-of-bounds access, in which case a `try/catch` would probably be better.

Fixes #881.