Christopher Faylor <me.cygwin2012@cgf.cx>

* fhandler.h (wait_return): Add overlapped_nullread.
* fhandler.cc (fhandler_base_overlapped::wait_overlapped): Detect zero-byte
read as a special case.
(fhandler_base_overlapped::raw_read): Keep looping when zero-byte read detected
without EOF.
(fhandler_base_overlapped::raw_write): Quiet gcc warning by adding
overlapped_nullread to switch statement even though it will never actually be
hit.

winsup/cygwin/fhandler.h

@@ -629,6 +629,7 @@ protected:
     overlapped_unknown = 0,
     overlapped_success,
     overlapped_nonblocking_no_data,
+    overlapped_nullread,
     overlapped_error
   };
   bool io_pending;