writefile: reuse SDNA between writes

Avoids decoding the SDNA string every undo step.

source/blender/windowmanager/intern/wm_init_exit.c

@@ -40,6 +40,7 @@
 
 #include "MEM_guardedalloc.h"
 
+#include "DNA_genfile.h"
 #include "DNA_scene_types.h"
 #include "DNA_userdef_types.h"
 #include "DNA_windowmanager_types.h"
@@ -584,6 +585,8 @@ void WM_exit_ext(bContext *C, const bool do_python)
 	
 	GHOST_DisposeSystemPaths();
 
+	DNA_sdna_current_free();
+
 	BLI_threadapi_exit();
 
 	BKE_blender_atexit();