[css-values-4] Add missing final return step to calc simplification (#11573)

Without this, it is possible to run off the end of the algorithm if
`root` is a math-function node without enough information to compute
its value.

Fixes #11572.

Author: AtkinsSJ

css-values-4/Overview.bs

@@ -4938,6 +4938,8 @@ Simplification</h4>
 
 			5. Return |root|.
 
+		10. Return |root|.
+
 	</div>