(svn r27989) -Fix (r27985): VA2 optimisation failed in various special cases:

- nvar=0 is meant to return the calculated result.
  - Missing references resolve to NULL and got identified with the default result.
  - Missing 'break' broke overlapping cases.
  - Splitting into non-overlapping cases could result in more than 256 cases.

src/newgrf_spritegroup.cpp

@@ -252,7 +252,7 @@ const SpriteGroup *DeterministicSpriteGroup::Resolve(ResolverObject &object) con
 
 	object.last_value = last_value;
 
-	if (this->num_ranges == 0) {
+	if (this->calculated_result) {
 		/* nvar == 0 is a special case -- we turn our value into a callback result */
 		if (value != CALLBACK_FAILED) value = GB(value, 0, 15);
 		static CallbackResultSpriteGroup nvarzero(0, true);