Change to allow MSVC2012 x64 build

[gemmell]

I'm compiling using VisualStudio 2012 for x64.

It compiled OK for the most part except for:

1>ClCompile:

1>  crash_generation_app.cc

1>crash_generation_app.cc(365): error C2065: 'GWL_HINSTANCE' : undeclared identifier

HINSTANCE instance = (HINSTANCE)GetWindowLong(wnd, GWL_HINSTANCE);

According to the docs GetWindowLong has been superseded  by GetWindowLongPtr (for support on 64bit systems). I haven't delved into the implications, but just changing that one line to:

HINSTANCE instance = (HINSTANCE)GetWindowLongPtr(wnd, GWLP_HINSTANCE);

At least allows it to compile. The GetWindowLongPtr doco says "When compiling for 32-bit Windows, GetWindowLongPtr is defined as a call to the GetWindowLong function." So it should be safe for 32bit compilations.