Audio mixer fix for number of frames not being a multipler of blocksize

When the desired number of frames is not a multipler of blocksize,
process__genericNoResampling will ask for 0 frames and crash.

Fix it.

BUG: 70902559

Test: cts-tradefed run cts -m CtsMediaTestCases -t
android.media.cts.AudioTrackLatencyTest#testOutputLowLatency

Change-Id: Id2ce771ee8a4e49fe6ea82e500017324c8918a73
(cherry picked from commit e38b72e410def6c44f090d149879cb620fcc7955)