Handler segfaults itself when using multiple v8 isolates

[laverdet]

In segfault-handler.cpp:113 the only check to determine if you're currently in the nodejs isolate is if (Isolate::GetCurrent()) which is not correct in multi-isolate environments (see my project: isolated-vm). So if a segfault is hit while in a different isolate the segfault handler will segfault itself when it tries to call a callback belonging to a different isolate.

It should be simple enough to grab a reference to the current isolate when the module is loaded and then compare against that in the segfault handler.