lavf: ignore attachment streams for interleaving purposes

Those streams should never get any packets by definition.

libavformat/options.c

@@ -19,6 +19,7 @@
  */
 #include "avformat.h"
 #include "avio_internal.h"
+#include "internal.h"
 #include "libavutil/opt.h"
 
 /**
@@ -100,6 +101,13 @@ AVFormatContext *avformat_alloc_context(void)
     ic = av_malloc(sizeof(AVFormatContext));
     if (!ic) return ic;
     avformat_get_context_defaults(ic);
+
+    ic->internal = av_mallocz(sizeof(*ic->internal));
+    if (!ic->internal) {
+        avformat_free_context(ic);
+        return NULL;
+    }
+
     return ic;
 }