Bugfix #24535: File saved with NLA Strip in Tweakmode crashes on reload

Now the active strip doesn't just get cleared on fileload, but is relinked properly. 

I had originally intended that files shouldn't be able to be saved with NLA data still in Tweakmode, but this turns out to be a bit more troublesome to get working as that would make undo keep popping out of this mode too.

Also reverting 32743 (bugfix for 24418), which was a hack around this.

source/blender/blenkernel/intern/anim_sys.c

@@ -1641,11 +1641,6 @@ static void animsys_evaluate_nla (PointerRNA *ptr, AnimData *adt, float ctime)
 			
 			if ((nlt) && !(adt->flag & ADT_NLA_EDIT_NOMAP)) {
 				/* edit active action in-place according to its active strip, so copy the data  */
-				
-				/* this is cleared on undo */
-				if(adt->actstrip == NULL) {
-					adt->actstrip= BKE_nlastrip_find_active(nlt);
-				}
 				memcpy(&dummy_strip, adt->actstrip, sizeof(NlaStrip));
 				dummy_strip.next = dummy_strip.prev = NULL;
 			}

source/blender/blenloader/intern/readfile.c

@@ -1976,9 +1976,13 @@ static void direct_link_animdata(FileData *fd, AnimData *adt)
 	link_list(fd, &adt->nla_tracks);
 	direct_link_nladata(fd, &adt->nla_tracks);
 	
-	/* clear temp pointers that may have been set... */
-	// TODO: it's probably only a small cost to reload this anyway...
-	adt->actstrip= NULL;
+	/* relink active strip - even though strictly speaking this should only be used
+	 * if we're in 'tweaking mode', we need to be able to have this loaded back for
+	 * undo, but also since users may not exit tweakmode before saving (#24535)
+	 */
+	// TODO: it's not really nice that anyone should be able to save the file in this
+	//		state, but it's going to be too hard to enforce this single case...
+	adt->actstrip= newdataadr(fd, adt->actstrip);
 }	
 
 /* ************ READ MOTION PATHS *************** */