Fix NuMediaExtractor::readSampleData buffer Handling

readSampleData() did not initialize buffer before filling it,
leading to OOB memory references. Correct and clarify the book
keeping around output buffer management.

Bug: 275418191
Test: CtsMediaExtractorTestCases w/debug messages
Change-Id: Ie744f118526f100d82a312c64f7c6fcf20773b6d