AudioPolicy: fix volume curve query for min/max index

Function to look up dB attenuation from index in volume curve
  didn't check against invalid index values.

Bug: 77731632
Test: cts-tradefed run cts-dev -m CtsMediaTestCases -t android.media.cts.AudioManagerTest#testGetStreamVolumeDb
Change-Id: Ia89cdbe84dcd586f54b5d08de3774054197aada0