Call SetPointerInInternalField(0, NULL) rather than
SetInternalField(0, Undefined()).

Fixes the following spurious NULL pointer dereference in debug builds:

  #0  0x03ad2821 in v8::internal::FixedArrayBase::length ()
  #1  0x03ad1dfc in v8::internal::FixedArray::get ()
  #2  0x03ae05dd in v8::internal::Context::global_object ()
  #3  0x03b6b87d in v8::internal::Context::builtins ()
  #4  0x03ae1871 in v8::internal::Isolate::js_builtins_object ()
  #5  0x03ab4fab in v8::CallV8HeapFunction ()
  #6  0x03ab4d4a in v8::Value::Equals ()
  #7  0x03b4f38b in CheckEqualsHelper ()
  #8  0x03ac0f4b in v8::Object::SetInternalField ()
  #9  0x06a99ddd in node::ObjectWrap::~ObjectWrap ()
  #10 0x06a8b051 in node::Buffer::~Buffer ()
  #11 0x06a8afbb in node::Buffer::~Buffer ()
  #12 0x06a8af5e in node::Buffer::~Buffer ()
  #13 0x06a9e569 in node::ObjectWrap::WeakCallback ()

by Sven Panne's suggestion, see [0] for details.

[0]: https://code.google.com/p/v8/issues/detail?id=2468

GetPointerFromInternalField() is deprecated now, we should use
GetAlignedPointerFromInternalField().

These explicit instantiations were added to make MSVC happy. It turns
out that some older versions of gcc and llvm now complain about duplicate
symbols, so we instantiate these templates only when MSVC is used.

Closes GH-2036