Audio HAL VTS: Sanitize prepareFor{Writing,Reading} input size

Return an error if framesCount or frameSize are null to avoid a division
by zero when calculating the buffer size.

The message queues are allocated with a buffer size but if two big they will
assert not return an error.
Thus take some margin on the buffer size check.

Note that both function should be refactored as 99% identical.

Test: vts-tradefed run vts --module VtsHalAudioV2_0Target
Test: call/play music/record/video...
Bug: 36311550
Change-Id: I0576e9016ef2e567c8d4e171c6237883d9865db9
Signed-off-by: Kevin Rocard <krocard@google.com>