Fix try catch crasher bug due to null else case.

test/codegen/try_catch.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 foo() {
   try {
     throw "hi there";
   } on String catch (e, t) {
   } catch (e, t) {
     rethrow;
   }
 }
+
 bar() {
   try {
     throw "hi there";
   } catch (e, t) {
   } on String catch (e, t) {
     // unreachable
     rethrow;
   }
 }
+
+baz() {
+  try {
+    throw "finally only";
+  } finally {
+    return true;
+  }
+}
+
+qux() {
+  try {
+    throw "on only";

[Jacob, 2015/03/27 17:04:30]

code that triggers a NPE without the fix.

+  } on String catch (e, t) {
+    // unreachable
+    rethrow;
+  }
+}
+
 main() {
   foo();
   bar();
+  baz();
+  qux();
 }