Only close an upvalue for a local if there actually is one for it.

test/language/closure/unused_later_closure.wren

@@ -0,0 +1,19 @@
+// This is a regression test. When closing upvalues for discarded locals, it
+// wouldn't make sure it discarded the upvalue for the correct stack slot.
+//
+// Here we create two locals that can be closed over, but only the first one
+// actually is. When "b" goes out of scope, we need to make sure we don't
+// prematurely close "a".
+var closure
+
+{
+  var a = "a"
+
+  {
+    var b = "b"
+    closure = Fn.new { a }
+    if (false) Fn.new { b }
+  }
+
+  System.print(closure.call()) // expect: a
+}