input/rewind: fix double free bug

Duplicate the "mime" attribute of the inner input_stream object,
instead of copying the pointer.

src/input/rewind_input_plugin.c

@@ -244,4 +244,5 @@ input_rewind_open(struct input_stream *is)
 	   input stream */
 	is->plugin = &rewind_input_plugin;
 	is->data = c;
+	is->mime = g_strdup(c->input.mime);
 }