x86/nHVM: avoid NULL deref during INVLPG intercept handling

When intercepting (or emulating) L1 guest INVLPG, the nested P2M
pointer may be (is?) NULL, and hence there's no point in calling
p2m_flush(). In fact doing so would cause a dereference of that NULL
pointer at least in the ASSERT() right at the beginning of the
function.

While so far nothing supports hap_invlpg() being reachable from the
INVLPG intercept paths (only INVLPG insn emulation would lead there),
and hence the code in question (added by dd6de3ab99 ["Implement
Nested-on-Nested"]) appears to be dead, this seems to be the change
which can be agreed on as an immediate fix. Ideally, however, the
problematic code would go away altogether. See thread at
lists.xenproject.org/archives/html/xen-devel/2016-01/msg03762.html.
   
Reported-by: 刘令 <liuling-it@xxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/mm/hap/hap.c
+++ b/xen/arch/x86/mm/hap/hap.c
@@ -687,7 +687,8 @@ static bool_t hap_invlpg(struct vcpu *v,
          * Must perform the flush right now or an other vcpu may
          * use it when we use the next VMRUN emulation, otherwise.
          */
-        p2m_flush(v, vcpu_nestedhvm(v).nv_p2m);
+        if ( vcpu_nestedhvm(v).nv_p2m )
+            p2m_flush(v, vcpu_nestedhvm(v).nv_p2m);
         return 1;
     }