player/Thread: merge two mutex locks

src/player/Thread.cxx

@@ -625,14 +625,14 @@ Player::SeekDecoder() noexcept
 				where = total_time;
 		}
 
+		const std::lock_guard<Mutex> lock(pc.mutex);
+
 		try {
 			const PlayerControl::ScopeOccupied occupied(pc);
 
-			const std::lock_guard<Mutex> lock(pc.mutex);
 			dc.Seek(where + start_time);
 		} catch (...) {
 			/* decoder failure */
-			const std::lock_guard<Mutex> lock(pc.mutex);
 			pc.SetError(PlayerError::DECODER,
 				    std::current_exception());
 			pc.CommandFinished();