Add assertions that the video cannot change on disk before writing

Trying to catch cases like these, which will now create bad output instead of failing because of #14. 

Moviepy will throw warning "Using the last valid frame instead."

```python
from moviepy.editor import VideoFileClip

testClip1 = VideoFileClip("test.mp4")
testClip2 = VideoFileClip("test.mp4").subclip(3, None)

testClip2.to_videofile("test.mp4")
testClip1.to_videofile("test.mp4")
```