Check buffer size after read

Ensure the WAV extractor never outputs a partial sample, which
would trigger a CHECK in NuPlayerRenderer:onDrainAudioQueue().
Also ensure it never emits a partial frame (e.g. only a left
sample for a stereo file).

Bug: 133824968
Test: play the corrupted file and there is no NE
Change-Id: I9143f859c55351e06126e4ad8f33f5752113d7f4