std.process: Fix spawnProcess in Windows GUI programs (issue #11180)

The old code also didn't check if `GetHandleInformation` succeeded, so this kills two birds with one stone (although we still don't throw on `GetHandleInformation` failure).

Note that although the documentation of `GetStdHandle` implies that the function will return `NULL` for GUI processes, this doesn't appear to be the case - GUI processes launched from console processes will have standard handles (`GetStdHandle(STD_INPUT_HANDLE)` will return 3), however those handles will be invalid (`GetHandleInformation` will fail for them). So this fix covers both cases, even though it ignores other reasons for `GetHandleInformation` to fail.