hwclock: do not create a zero adjfile

hwclock: do not create a zero adjfile

When hwclock --hctosys is started very early during the system startup,
with / still mounted read-only, and there was no /etc/adjtime file,
hwclock fails creating a default adjfile full of zeroes, and prints an
error message. I believe that such zero adjfile is not necessary,
because it means exactly the same as no adjfile at all.

The attached patch prevents creation of a zero adjfile, of course unless
something gets changed (this never happens during a --hctosys).

Signed-off-by: Alain Guibert <alguibert+ulng@free.fr>