io_win32_unittest: remove incorrect error check
io_win32_unittest: remove incorrect error check

Unlike GetEnvironmentVariableW,
GetCurrentDirectoryW doesn't set
ERROR_INSUFFICIENT_BUFFER.

google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf