x86/xen: silence smatch warning in pmu_msr_chk_emulated()

Commit 8714f7bc ("xen/pv: add fault recovery control to pmu msr
accesses") introduced code resulting in a warning issued by the smatch
static checker, claiming to use an uninitialized variable.

This is a false positive, but work around the warning nevertheless.

Fixes: 8714f7bc ("xen/pv: add fault recovery control to pmu msr accesses")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>