Fix NullPointerException (#134)

* Fix NullPointerException

https://github.com/natario1/Transcoder/issues/133

Fix NullPointerException with ClipDataSource(FilePathDataSource())
Exception:
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.media.MediaMetadataRetriever.extractMetadata(int)' on a null object reference
at com.otaliastudios.transcoder.source.DefaultDataSource.getDurationUs(DefaultDataSource.java:276)
at com.otaliastudios.transcoder.source.ClipDataSource.getSourceDurationUs(ClipDataSource.java:23)
at com.otaliastudios.transcoder.source.ClipDataSource.(ClipDataSource.java:18)

* Fix 0 duration audio output issue.

https://github.com/natario1/Transcoder/issues/135

Author: DamonChen117

lib/src/main/java/com/otaliastudios/transcoder/source/FilePathDataSource.java

@@ -38,6 +38,7 @@ public void initialize() {
         } catch (IOException e) {
             throw new RuntimeException(e);
         }
+        super.initialize();
     }
 
     @Override
@@ -46,6 +47,7 @@ public void deinitialize() {
         // This means that we must also recreate the underlying source.
         mDescriptorSource.deinitialize();
         try { mStream.close(); } catch (IOException ignore) { }
+        super.deinitialize();
     }
 
     @Override

lib/src/main/java/com/otaliastudios/transcoder/source/TrimDataSource.java

@@ -65,6 +65,11 @@ public void initialize() {
     public long getDurationUs() {
         return trimDurationUs + extraDurationUs;
     }
+    
+    @Override
+    public long getPositionUs() {
+        return super.getPositionUs() - trimStartUs;
+    }
 
     @Override
     public boolean canReadTrack(@NonNull TrackType type) {