audiounit: guard counter with stream mutex similar to stream init. (#411)

Author: achronop

src/cubeb_audiounit.cpp

@@ -2683,12 +2683,10 @@ audiounit_stream_destroy(cubeb_stream * stm)
   dispatch_sync(stm->context->serial_queue, ^() {
     auto_lock lock(stm->mutex);
     audiounit_close_stream(stm);
+    assert(stm->context->active_streams >= 1);
+    stm->context->active_streams -= 1;
   });
 
-  auto_lock context_lock(stm->context->mutex);
-  assert(stm->context->active_streams >= 1);
-  stm->context->active_streams -= 1;
-
   LOG("Cubeb stream (%p) destroyed successful.", stm);
   delete stm;
 }