Fix null terminator and charset issues in heif implementation

[payton]

Closes #362

As per ISO documentation (in class comments), the modified fields are supposed to be null terminated and/or specify the UTF-8 charset.

[payton]

Can @lzzcg and/or @endy1106 validate these changes fix the first two points in #362? I was able to reproduce and fix the first issue, but I was unable to reproduce the second issue. I still made the changes because ISO documentation states the previous implementation was incorrect (not specifying Charset).

[payton]

@drewnoakes , once they can confirm their issues are resolved, feel free to merge this PR if you see fit.

[drewnoakes]

Changes look good. Ran it on the one HEIC image we have from the linked issue.

Do you know where we can get some more HEIC images? There's a Nokia test set online but they're copyrighted.

[payton]

I was just looking into that... Since Apple has now made the switch from JPEG to HEIF, we should be able to get some good sample images that way. By default, I believe photos are converted to JPEG on iPhone (when exported), but there is an option to keep the original file. I don't have an iPhone, but I can ask around to try this out.

https://www.softwarert.com/transfer-photos-iphone-to-computer-heic-jpg/

[payton]

I'm going to go ahead and merge this as it seems to be an important fix for HEIF support.

@lzzcg and/or @endy1106 please comment on the issue if there is something that should be modified/added.