Movie processing made more stable while cancelling

* Checking more statuses for GPUImageMovie, as reader can be failed or
cancelled, but this is not handled properly
* Checking inputs' statuses before appending buffers in 
GPUImageMovieWriter, also checking blocks before calling them

This changes became required, as I was implementing movie file processing for my project, and it turned out that GPUImageMovie cancelProcessing method tends to crash the app time after time, especially on slower devices. After these fixes cancelling process became much more stable and reliable.