Fix a crash in the extension system when sending response back

chrome/browser/extensions/extension_function.cc

Index: chrome/browser/extensions/extension_function.cc
===================================================================
--- chrome/browser/extensions/extension_function.cc	(revision 16294)
+++ chrome/browser/extensions/extension_function.cc	(working copy)
@@ -26,7 +26,9 @@
 
 const std::string AsyncExtensionFunction::GetResult() {
   std::string json;
-  JSONWriter::Write(result_.get(), false, &json);
+  // Some functions might not need to return any results.
+  if (result_.get())
+    JSONWriter::Write(result_.get(), false, &json);
   return json;
 }