export: crash if nullptr passed

The export module is now intended to be initialized always, although
not exporting at that moment, so assert the state presence for
export_audio/export_video.

src/export.c

@@ -35,6 +35,7 @@
  * EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <assert.h>
 #include <dirent.h>
 #include <errno.h>            // for errno, EEXIST
 #include <limits.h>
@@ -340,9 +341,7 @@ static void process_messages(struct exporter *s) {
 
 void export_audio(struct exporter *s, struct audio_frame *frame)
 {
-        if(!s){
-                return;
-        }
+        assert(s != NULL);
 
         process_messages(s);
 
@@ -355,9 +354,7 @@ void export_audio(struct exporter *s, struct audio_frame *frame)
 
 void export_video(struct exporter *s, struct video_frame *frame)
 {
-        if(!s){
-                return;
-        }
+        assert(s != NULL);
 
         process_messages(s);