xcb: Deal with case of getenv() returning NULL.
xcb: Deal with case of getenv() returning NULL.

At least on Windows, getenv() returns NULL when the environment variable is not
found.  This causes a problem inside of "xcb_nvidia.cpp" when a C++ std::string
is assigned the return value of getenv().

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools