swresample: add swr_is_initialized()

Idea-from/based-on: 7e86c27b4e
Reviewed-by: ubitux
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswresample/swresample.h

@@ -165,6 +165,13 @@ struct SwrContext *swr_alloc(void);
  */
 int swr_init(struct SwrContext *s);
 
+/**
+ * Check whether an swr context has been initialized or not.
+ *
+ * @return positive if it has been initialized, 0 if not initialized
+ */
+int swr_is_initialized(struct SwrContext *s);
+
 /**
  * Allocate SwrContext if needed and set/reset common parameters.
  *