Add moov mdat stream check

[okyanusoz]

Description

I added a check for the mdat stream not to be null and warn the user about it because it can sometimes cause exceptions, like below:

[DenizUgur]

This would make avcC writing in mixed endianness. You should specify endianness on DataStream (in your application).

For the mdat issue. That probably happens when you're not using createFile(true) which would instruct the library to discard the mdat (when it's set to false). If you could revise the PR to check for that and warn if we should be expecting data in mdat but it's not there, that would be better.

[okyanusoz]

You're right about the moov and avCC part. I'll amend the PR for that.

For reference, the reason why avcC was failing was because I used DataStream.BIG_ENDIAN, which doesn't exist in the latest version. I modified my code to use Endianness.BIG_ENDIAN and it worked.