RtpToNtpEstimator:: Add a DCHECK to avoid div-by-0

The division that follows the added DCHECK should not be div-by-0,
since when params_.calculated is true, params_.frequency_khz would
have been updated (with something non-zero).

BUG=none

Review-Url: https://codereview.webrtc.org/3006003002
Cr-Original-Commit-Position: refs/heads/master@{#19671}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: d56ef8085718e15c497b0d9c60baa1a4640420fa