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