libaudiohal: Fix crash due to uninitialized field

EffectHalHidl::mEfGroup could be left uninitialized leading
to a crash when trying to delete the event flag.

Bug: 64495982
Bug: 64553871
Test: see repro steps on b/64553871
Change-Id: Ifd2f76f2d8cc845ad46949c18050655bea946a42

media/libaudiohal/EffectHalHidl.cpp

@@ -40,7 +40,7 @@ using ::android::hardware::Return;
 namespace android {
 
 EffectHalHidl::EffectHalHidl(const sp<IEffect>& effect, uint64_t effectId)
-        : mEffect(effect), mEffectId(effectId), mBuffersChanged(true) {
+        : mEffect(effect), mEffectId(effectId), mBuffersChanged(true), mEfGroup(nullptr) {
 }
 
 EffectHalHidl::~EffectHalHidl() {