QuickTime: reset stream if an invalid atom length is found

This is particularly necessary if the "header" is at the end of the file.

Also clarifies a variable name that's been annoying me for a while, and
fixes a couple of typos.

Fixes #12218.
Merge pull request #1101 from melissalinkert/rebased/develop/ticket-12218

QuickTime: fix header/footer parsing bug (rebased onto develop)