Deduce the duration if it isn't already present.

src/main/java/org/forkalsrud/album/video/FlvFilter.java

@@ -188,6 +188,7 @@ public class FlvFilter extends OutputStream {
         if (currentBox != null) {
             throw new IOException("Premature end of file");
         }
+        metadata.setDurationFromLastTimestamp();
     }
 
     /**

src/main/java/org/forkalsrud/album/video/FlvMetadata.java

@@ -438,9 +438,19 @@ public class FlvMetadata {
     }
 
     public void setDuration(String durationStr) {
-        this.duration.set(Double.parseDouble(durationStr));
+        setDuration(Double.parseDouble(durationStr));
     }
     
+    public void setDuration(double seconds) {
+        this.duration.set(Double.valueOf(seconds));
+    }
+
+    public void setDurationFromLastTimestamp() {
+        if (duration.value == null || duration.value.doubleValue() <= 0) {
+            setDuration(lasttimestamp.value / 1000d);
+        }
+    }
+
     public void setDimension(Dimension d) {
         this.width.set(Double.valueOf(d.getWidth()));
         this.height.set(Double.valueOf(d.getHeight()));