mixer: advance position before mixing

Sometimes, the play position in stream is behind the out stream, mostly
because of timing issue. In this case, the mixer will produce invalid
packets which in return will lead to looping on an invalid packet in
the audio_drv.

Issue #1666.

repos/os/src/server/mixer/mixer.cc

@@ -292,11 +292,11 @@ class Audio_out::Mixer : public Thread<1024 * sizeof(addr_t)>
 					continue;
 				}
 
-				_mix();
-
 				/* advance position of clients */
 				_advance_position();
 
+				_mix();
+
 				if (!_left.stream()->empty())
 					_wait_for_progress();
 				else