drm/i915/gvt: Assign NULL to the pointer after memory free.

The klocwork static code analyzer complains about using pointer after
being freed, because further we pass it to the gvt_vgpu_err() function.
Assign pointer to be NULL intentionaly, to meet requirements of the code
analyzer.

This patch fixed the issue #648 reported as error by klocwork.
Acked-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Aleksei Gimbitskii <aleksei.gimbitskii@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>

drivers/gpu/drm/i915/gvt/gtt.c

@@ -1117,6 +1117,7 @@ static struct intel_vgpu_ppgtt_spt *ppgtt_populate_spt_by_guest_entry(
 
 err_free_spt:
 	ppgtt_free_spt(spt);
+	spt = NULL;
 err:
 	gvt_vgpu_err("fail: shadow page %p guest entry 0x%llx type %d\n",
 		     spt, we->val64, we->type);