Minor cleanup for win32stat.c.

Use GetLastError(), rather than assuming that CreateFile() failure
must map to ENOENT.  Noted by Michael Paquier.

Discussion: https://postgr.es/m/CAC+AXB0g44SbvSpC86o_1HWh8TAU2pZrMRW6tJT-dkijotx5Qg@mail.gmail.com

src/port/win32stat.c

@@ -204,8 +204,10 @@ _pgstat64(const char *name, struct stat *buf)
 					   NULL);
 	if (hFile == INVALID_HANDLE_VALUE)
 	{
+		DWORD		err = GetLastError();
+
 		CloseHandle(hFile);
-		errno = ENOENT;
+		_dosmaperr(err);
 		return -1;
 	}