Move token signature verification after fields

[palsivertsen]

Fixes #174

Token signature validation can be an expensive operation.
For tokens with invalid fields it is not necessary to check the
signature and it is therfore moved to the bottom.

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[palsivertsen]

I signed it!

[hiranya911]

Code changes LGTM. Thanks @palsivertsen for putting this together. I'm wondering why the CLA check hasn't passed yet. Can you review the following instructions, and see everything is in order:

It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

[palsivertsen]

There where some issues with the CLA. I updated the document and CLA is pending. Hopefully processed today.

[palsivertsen]

@hiranya911 can you recheck the CLA?

[googlebot]

CLAs look good, thanks!

[hiranya911]

LGTM