Small fixes to make untrunc work with a video containing lots of 0-bytes at the end

[davitf]

atom.cpp:

• Fix buffer reuse logic in getFragment(). (speeds up considerably when skipping over large blocks of zeros)
• Fix "Out of buffer" crash in readInt() when reading near the end of the file. (needed for a previous version of the fix; probably a no-op now, but seems safer for the future)

mp4.cpp:

• Fix crash when the offset is < 4 bytes from the end of the block, i.e. a full int cannot be read.

[anthwlock]

I would appreciate it if you could find some time to test if this bug is still present in my master

[davitf]

Hi anthwlock, I've tested it. The process completes successfully, but for some reason the resulting video does not play correctly... I could send you my reference and broken videos if you'd like to test it.

[anthwlock]

Yes, that would be great! Open a Issue on my repository please.