Spanify the memcpy in RTCEncodedVideoFrameDelegate::CreateDataBuffer

In addition to being bounds-safe, it avoids an issue with memcpy and
empty inputs (see https://davidben.net/2024/01/15/empty-slices.html)

This fixes the UBSan warning below:
https://ci.chromium.org/ui/labs/p/chromium/inv/build-8738203517420344721/test/ninja%3A%2F%2Fthird_party%2Fblink%2Frenderer%2Fcontroller%3Ablink_unittests%2FRTCEncodedVideoFrameTest.PassWebRTCDetachesFrameData/variant/603704313003e25d?resultIndex=0

Bug: 40248746
Change-Id: I585728564d9f389d268b66a6730fae5a7f9eeb03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5827021


Auto-Submit: David Benjamin <davidben@chromium.org>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1349426}