fix isValid() for VAAPI frame (#141)

* fix for AVAPI frame format
* valid values for VASurfaceID is > 0

Author: lukasz-misek

src/frame.h

@@ -264,7 +264,12 @@ class Frame : public FFWrapperPtr<AVFrame>
 
     bool isComplete() const { return m_isComplete; }
 
-    bool isValid() const { return (!isNull() && m_raw->data[0] && m_raw->linesize[0]); }
+    bool isValid() const {
+         return (!isNull() &&
+             ((m_raw->data[0] && m_raw->linesize[0]) ||
+             ((m_raw->format == AV_PIX_FMT_VAAPI) && ((intptr_t)m_raw->data[3] > 0)))
+         );
+    }
 
     operator bool() const { return isValid() && isComplete(); }