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:

rust-ffmpeg/src/software/resampling/context.rs

Line 58 in 0bcd455

if ptr.is_null() {

[barzamin]

These is_null checks were fixed in dc6ace9.