Fix ClipDataSource initialization

Author: natario1

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

@@ -15,6 +15,11 @@ public ClipDataSource(@NonNull DataSource source, long clipStartUs) {
     public ClipDataSource(@NonNull DataSource source, long clipStartUs, long clipEndUs) {
         super(new TrimDataSource(source,
                 clipStartUs,
-                source.getDurationUs() - clipEndUs));
+                getSourceDurationUs(source) - clipEndUs));
+    }
+
+    private static long getSourceDurationUs(@NonNull DataSource source) {
+        if (!source.isInitialized()) source.initialize();
+        return source.getDurationUs();
     }
 }