build: fix level 1 msvc warnings

[invertego]

Level 1 warnings are considered "severe" and, unsurprisingly, turning them on revealed some true bugs.

• file-buffer.hpp: What should have been a zero extend followed by a complement was instead a complement followed by a zero extend, inadvertently truncating the file seek offset to 32 bits.
• file-map.cpp: What should have been a nullptr check after CreateFileMapping() was instead a check for INVALID_HANDLE_VALUE. The warning pointing me toward this was not technically about the misuse but rather the redefinition of INVALID_HANDLE_VALUE in nall. I've removed this and any reference to INVALID_HANDLE_VALUE except for error checks immediately after calls to the few APIs that return it.