Fix 2 for T98700: Crash when recursively nesting NLA meta strips

When searching for the active NLA strip, avoid overwriting the found strip
pointer with NULL if it was already found in a previous iteration.

The active strip is searched for while looping over the NLA tracks. If the
active strip was found on a previous track, and not on the current track,
this would effectively set `actstrip = NULL`. This is now avoided.

Another benefit is that the search for the active strip is stopped as soon
as it's found, which should increase performance a tiny bit.

source/blender/blenkernel/intern/nla.c

@@ -311,7 +311,11 @@ static void update_active_track(AnimData *adt_dest, const AnimData *adt_source)
     if (track_source == adt_source->act_track) {
       adt_dest->act_track = track_dest;
     }
-    update_active_strip(adt_dest, track_dest, adt_source, track_source);
+
+    /* Only search for the active strip if it hasn't been found yet. */
+    if (adt_dest->actstrip == NULL && adt_source->actstrip != NULL) {
+      update_active_strip(adt_dest, track_dest, adt_source, track_source);
+    }
 
     track_dest = track_dest->next;
   }