x86/tsc: Validate cpumask pointer before accessing it

0-day testing encountered a NULL pointer dereference in a cpumask access
from tsc_store_and_check_tsc_adjust().

This happens when the function is called on the boot CPU and the topology
masks are not yet available due to CPUMASK_OFFSTACK=y.

Add a NULL pointer check for the mask pointer. If NULL it's safe to assume
that the CPU is the boot CPU and the first one in the package.

Fixes: 8b223bc7

 ("x86/tsc: Store and check TSC ADJUST MSR")
Reported-by: kernel test robot <xiaolong.ye@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>