x86/time: Fix UBSAN failure in __update_vcpu_system_time()

As reported:

  (XEN) ================================================================================
  (XEN) UBSAN: Undefined behaviour in arch/x86/time.c:1542:32
  (XEN) member access within null pointer of type 'union vcpu_info_t'
  (XEN) ----[ Xen-4.19-unstable  x86_64  debug=y ubsan=y  Not tainted ]----
  ...
  (XEN) Xen call trace:
  (XEN)    [<ffff82d040345036>] R common/ubsan/ubsan.c#ubsan_epilogue+0xa/0xd2
  (XEN)    [<ffff82d0403456e8>] F __ubsan_handle_type_mismatch+0x133/0x49b
  (XEN)    [<ffff82d040345b4a>] F __ubsan_handle_type_mismatch_v1+0xfa/0xfc
  (XEN)    [<ffff82d040623356>] F arch/x86/time.c#__update_vcpu_system_time+0x212/0x30f
  (XEN)    [<ffff82d040623461>] F update_vcpu_system_time+0xe/0x10
  (XEN)    [<ffff82d04062389d>] F arch/x86/time.c#local_time_calibration+0x1f7/0x523
  (XEN)    [<ffff82d0402a64b5>] F common/softirq.c#__do_softirq+0x1f4/0x31a
  (XEN)    [<ffff82d0402a67ad>] F do_softirq+0x13/0x15
  (XEN)    [<ffff82d0405a95dc>] F arch/x86/domain.c#idle_loop+0x2e0/0x367
  (XEN)
  (XEN) ================================================================================

GCC 10 at least doesn't consider it valid to derive a pointer from vcpu_info()
prior to checking that the underlying map pointer is good.

Reorder actions so the map pointer is checked first.

Fixes: 20279afd73 ("x86: split populating of struct vcpu_time_info into a separate function")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>

xen/arch/x86/time.c

@@ -1539,12 +1539,14 @@ static void collect_time_info(const struct vcpu *v,
 
 static void __update_vcpu_system_time(struct vcpu *v, int force)
 {
-    struct vcpu_time_info *u = &vcpu_info(v, time), _u;
+    struct vcpu_time_info *u, _u;
     const struct domain *d = v->domain;
 
     if ( !v->vcpu_info_area.map )
         return;
 
+    u = &vcpu_info(v, time);
+
     collect_time_info(v, &_u);
 
     /* Don't bother unless timestamp record has changed or we are forced. */