Improve golint coverage

[wafuwafu13]

@sunfish-shogi

Related #23 Golint

I fixed golint warning except should have comment or be unexported.
https://goreportcard.com/report/github.com/abema/go-mp4#golint

[wafuwafu13]

if block ends with a return statement, so drop this else and outdent its block (move short variable declaration to its own line if necessary) warning also occured here, but I couldn't fix because of no new variables on left side of :=.

go-mp4/marshaller.go

Lines 264 to 269 in d23f5a8

	if dst, err := boxType.New(ctx); err != nil {
	return nil, 0, err
	} else {
	n, err := Unmarshal(r, payloadSize, dst, ctx)
	return dst, n, err
	}

	dst, err := boxType.New(ctx)
	if err != nil {
		return nil, 0, err
	}
	n, err := Unmarshal(r, payloadSize, dst, ctx) // no new variables on left side of :=
	return dst, n, err

[sunfish-shogi]

Thank you for your pull request!

#90 (comment)
In this case, you can use "=" operator instead of ":=".
Should I wait for your additional changes?

[wafuwafu13]

Thank you for teaching me :)
I fixed it at f9ab551 .