Fix UB when comparing two empty webrtc::Buffer objects

operator== invoked std::memcmp(nullptr, nullptr, 0) when both buffers
were empty, which is undefined behavior and triggers a ubsan crash. Fix
the issue by returning early if both buffers have size 0.

Bug: webrtc:42224551
Change-Id: I073171f57640e20115361146ebbe89e1781be0af
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/472020
Commit-Queue: Joachim Reiersen <joachimr@meta.com>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47694}