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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:943fc12219b21d2a98f0ddc070b9b316a6f5d412)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:84c69bca81175feb2fd97ebb22e432ee41572786)
Merged-In: Ie744f118526f100d82a312c64f7c6fcf20773b6d
Change-Id: Ie744f118526f100d82a312c64f7c6fcf20773b6d