Crash on get user prefs

[anaisbetts]

• Electron version: 1.3.something
• Operating system: OS X 10.10.5

Saw this in a user report, looks like GetUserDefault can return bogus values

0 com.github.electron.framework 0x00000001009b7304 base::Value::GetType() const + 4
1 com.github.electron.framework 0x00000001009b6c15 atom::V8ValueConverter::ToV8ValueImpl(v8::Isolate*, base::Value const*) const + 37
2 com.github.electron.framework 0x00000001009b6bbd atom::V8ValueConverter::ToV8Value(base::Value const*, v8::Local<v8::Context>) const + 77
3 com.github.electron.framework 0x00000001009b86cb mate::Converter<base::ListValue, void>::ToV8(v8::Isolate*, base::ListValue const&) + 59
4 com.github.electron.framework 0x00000001009c5ee9 v8::Local<v8::Value> mate::ConvertToV8<base::ListValue>(v8::Isolate*, base::ListValue const&) + 9
5 com.github.electron.framework 0x0000000100909e8a atom::api::SystemPreferences::GetUserDefault(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) + 1130

[kevinsawicki]

Fixed by #7993