support catch without error (#58)

index.js

@@ -132,7 +132,7 @@ function findGlobals(source, options) {
     },
     'Class': declareClass,
     'TryStatement': function (node) {
-      if (node.handler === null) return;
+      if (node.handler === null || node.handler.param === null) return;
       node.handler.locals = node.handler.locals || Object.create(null);
       declarePattern(node.handler.param, node.handler);
     },

test/fixtures/catch-without-error.js

@@ -0,0 +1 @@
+try{}catch{}

test/index.js

@@ -43,6 +43,9 @@ test('assign_implicit.js - assign from an implicit global', function () {
 test('catch-pattern.js - pattern in catch', function () {
   assert.deepEqual(detect(read('catch-pattern.js')), []);
 });
+test('catch-without-error.js - catch without error', function () {
+  assert.deepEqual(detect(read('catch-without-error.js')), []);
+});
 test('class.js - ES2015 classes', function () {
   assert.deepEqual(detect(read('class.js')).map(nameOf), ['G', 'OtherClass_', 'SuperClass', 'this'].sort());
 });