Remove redundant null pointer checks.
move header fields validation when they passed in rather when used.
Validate all used fields from the header.
Bug: webrtc:8335
Change-Id: I20b132c6fb8966e49a5414fe757f74d504f4a61a
Reviewed-on: https://webrtc-review.googlesource.com/7400
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20194}