AudioUnitManager: Call dispatch_group_leave on instantiation error

When AudioComponentInstantiate returns an error, the callback
returned early without calling dispatch_group_leave. This caused
waitForAudioUnit to block for the full 2s timeout on every
failing AU, even though the error was already known.

Author: xARSENICx

src/effects/backends/audiounit/audiounitmanager.mm

@@ -94,6 +94,7 @@
             qWarning() << "Could not instantiate Audio Unit"
                        << pManager->m_name << ":" << error
                        << "(Check https://www.osstatus.com for a description)";
+            dispatch_group_leave(pManager->m_instantiationGroup);
             return;
         }