fix a number of memory leaks

- some were with libav functions
- same were due to non-virtual destructors

src/ReaderBase.cpp

@@ -63,6 +63,9 @@ ReaderBase::ReaderBase()
 	parent = NULL;
 }
 
+ReaderBase::~ReaderBase() {
+}
+
 // Display file information
 void ReaderBase::DisplayInfo() {
 	cout << fixed << setprecision(2) << boolalpha;