NLA: add BLI_assert_msg() to check for assumption

`find_active_strip_from_listbase()` expects two lists of strips with an
equal number of items. This is now not only documented, but also checked
for in an assertion.

source/blender/blenkernel/intern/nla.c

@@ -252,6 +252,9 @@ static NlaStrip *find_active_strip_from_listbase(const NlaStrip *active_strip,
                                                  const ListBase /* NlaStrip */ *strips_source,
                                                  const ListBase /* NlaStrip */ *strips_dest)
 {
+  BLI_assert_msg(BLI_listbase_count(strips_source) == BLI_listbase_count(strips_dest),
+                 "Expecting the same number of source and destination strips");
+
   NlaStrip *strip_dest = strips_dest->first;
   LISTBASE_FOREACH (const NlaStrip *, strip_source, strips_source) {
     if (strip_dest == NULL) {