unreachable code

In the scanImage function in the decoder there's this [bit](https://github.com/hipstersmoothie/bmp-ts/blob/8ea7477757fce6ac0afee82462f7a21a3f280775/lib/decoder.ts#L471):

` const result = processPixel.call(this, x, line);
if (result === false) {
return;
}`

...but processPixel is defined as returning void, hence the if statement will never evaluate to true. Any idea what was intended with that code? I can submit a pr and fix it, but just glancing at it I'm not sure what the intent was. Thanks!