[Cast] Fix frame mismatch in WavMixerInputSource

Wrap the Audio Bus if num_frames is less than buffer frames.

Bug: b/175134526
Test: run cplay with config that filter group doesn't require resampler.
Change-Id: I477221ad202aa4af11fdb535da0c27203aaf560d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2585959
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Mingyu Kang <kenkangxgwe@google.com>
Cr-Commit-Position: refs/heads/master@{#835969}