Implement fhandler_dev_null::write to workaround a problem with NUL

Windows NUL device returns only the lower 32 bit of the number of
bytes written.  Implement a fake write function to ignore the underlying
NUL device.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/fhandler.cc

@@ -1529,6 +1529,15 @@ fhandler_dev_null::fhandler_dev_null () :
 {
 }
 
+ssize_t __stdcall
+fhandler_dev_null::write (const void *ptr, size_t len)
+{
+  /* Shortcut.  This also fixes a problem with the NUL device on 64 bit:
+     If you write > 4 GB in a single attempt, the bytes written returned
+     from by is numBytes & 0xffffffff. */
+  return len;
+}
+
 void
 fhandler_base::set_no_inheritance (HANDLE &h, bool not_inheriting)
 {