Improve code style in parse_mdhd.
Improve code style in parse_mdhd.

Use two-armed match instead of let ... = if { ... } else { ... };
This is shorter and easier to read.

Use an early return for error conditions. This could also be
a two-armed match; not sure which is better.

kinetiknz/mp4parse-rust,kinetiknz/mp4parse-rust,mozilla/mp4parse-rust