Add support for large-address aware applications

[m417z]

Large-address aware applications linked with the /LARGEADDRESSAWARE flag can have a user-mode address with its upper bit set. The current code sign-extends the pointer, e.g. 87aaeb58 to ffffffff87aaeb58, leading to an invalid pointer and likely to a crash.

I discovered this due to a bug report for Windhawk:
ramensoftware/windhawk#3

I came up with a small program which triggers the bug most of the time:
https://gist.github.com/m417z/7177d820252ab42f4d86c905589b6f05

Also, a similar notice regarding 32-bit to 64-bit pointer casting can be found here:

Assigning a pointer to the Buffer member will sign-extend the value if the code is compiled as 32-bits; this can break large-address aware applications running on systems configured with 4-Gigabyte Tuning or running under WOW64 on 64-bit Windows. Therefore, use the PtrToPtr64 macro when assigning pointers to Buffer.

[learn-more]

If this is ever called from x64 code, it will truncate the pointer

[m417z]

Yes, wow64ext is an x32-only library, but PTR_TO_DWORD64 might be helpful in generic code outside of the library. I changed the macro, should be fine now, thanks.

[therealdreg]

for me its ok, @rwfpl can you merge this PR?

[injertao]

PTR_TO_DWORD64 not applicable to hProcess
When hProcess = -1

[m417z]

@injertao good catch. Here's a PR to fix this: #22