audioflinger: fix regression in attachAuxEffect().

Commit 717e1286 introduced a regression in PlaybackThread::Track::attachAuxEffect()
when called with an effect ID of 0 to detach the auxiliary effect.

It is normal in this case that AudioFlinger::getEffectThread_l() returns 0.

Bug 6768757.

Change-Id: I7430bd1aad2f68da38f7c3e4794e7ad657bfc6be
1 file changed