Fix a bug in nested context code that occured when calling a function nested in

the outermost scope with a context frame from a function using a more nested
context frame.

tests/mini/nested21.d

@@ -0,0 +1,16 @@
+module nested21;
+
+extern(C) int printf(char*, ...);
+
+void main() {
+    int i = 42;
+    int foo() { return i; }
+    int bar() {
+        int j = 47;
+        int baz() { return j; }
+        return foo() + baz();
+    }
+    auto result = bar();
+    printf("%d\n", result);
+    assert(result == 42 + 47);
+}