estream: Fix IS_INVALID_FD.

* src/estream.c (IS_INVALID_FD): Simply compare to -1.

Perhaps, when it was written at first, use of HANDLE was in mind.
Now, we have different set of methods for Windows, and this macro
is only for integer fd access, no need of ifdef/endif.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>