CRAS: float_buffer - Fix readable count

Fix a bug in float_buffer when passing non-zero offset. Without
the fix, the result readable frames count could be incorrectly
larger and cause crash.

BUG=chromium:892608
TEST=unittest

Change-Id: I2d5f1724a032bc3264e4c178bf2b4e488686cde4
Reviewed-on: https://chromium-review.googlesource.com/1265215
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Hsinyu Chao <hychao@chromium.org>
Reviewed-by: Dylan Reid <dgreid@chromium.org>
3 files changed