decoder: initialize last timestamp

When always recording, the uninitialized value could break the overlay
filter, making one of the video sources not get filtered properly.

Change-Id: I137aa175f0ba5d88a660c6bcfa54f37bd65b7c9d

src/media/media_decoder.h

@@ -169,7 +169,7 @@ private:
     AVStream *avStream_ = nullptr;
     bool emulateRate_ = false;
     int64_t startTime_;
-    int64_t lastTimestamp_;
+    int64_t lastTimestamp_ {0};
 
     DeviceParams inputParams_;