Fix for potential NULL de-reference issue in Reverb

Fix for potential NULL de-reference issue in EffectReverb.cpp file

Test: android.media.cts.AudioEffectTest
Test: android.media.cts.AudioPreProcessingTest
Test: android.media.cts.BassBoostTest
Test: android.media.cts.EnvReverbTest
Test: android.media.cts.EqualizerTest
Test: android.media.cts.LoudnessEnhancerTest
Test: android.media.cts.PresetReverbTest
Test: android.media.cts.VirtualizerTest
Test: android.media.cts.VisualizerTest
Bug: 77731062
Change-Id: Ib293173bb2512553bacb9a35dcf0f82bb290ede8