fix: get scaling context && align error

[xinghun92]

• From the api doc of ffmpeg, sws_getContext will return null if an error occurs, otherwise a pointer to an allocated context.

• If the width is not divisible by the align, wrong data will be processed.

[YaLTeR]

Same ptr::is_null() => !ptr::is_null() should be applied for the resampling part too: https://github.com/meh/rust-ffmpeg/blob/0bcd4550b85580d02599d84b312251d930d1802f/src/software/resampling/context.rs#L58

[barzamin]

These is_null checks were fixed in dc6ace91ec73b14953b4157af2b1c4c2156c5354.