ipmi:ssif: Handle a possible NULL pointer reference

In error cases a NULL can be passed to memcpy.  The length will always
be zero, so it doesn't really matter, but go ahead and check for NULL,
anyway, to be more precise and avoid static analysis errors.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>