domains: don't crash if domain is set to null

Closes #4256

src/node.cc

@@ -1020,7 +1020,7 @@ MakeCallback(const Handle<Object> object,
   Local<Object> domain;
   Local<Function> enter;
   Local<Function> exit;
-  if (!domain_v->IsUndefined()) {
+  if (domain_v->IsObject()) {
     domain = domain_v->ToObject();
     if (domain->Get(disposed_symbol)->BooleanValue()) {
       // domain has been disposed of.
@@ -1042,7 +1042,7 @@ MakeCallback(const Handle<Object> object,
     return Undefined();
   }
 
-  if (!domain_v->IsUndefined()) {
+  if (domain_v->IsObject()) {
     exit = Local<Function>::Cast(domain->Get(exit_symbol));
     exit->Call(domain, 0, NULL);
   }

test/simple/test-regress-GH-4256.js

@@ -0,0 +1,25 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+process.domain = null;
+timer = setTimeout(function() {
+  console.log("this console.log statement should not make node crash");
+}, 1);