DM USB: xHCI: Fix a potential NULL pointer issue.

After excap pointer is assigned, it should be checked whether it's
possible to get assignment for NULL pointer or not. This patch
fixes this issue.

Tracked-On: #1479
Signed-off-by: Liang Yang <liang3.yang@intel.com>
Reviewed-by: Xiaoguang Wu <xiaoguang.wu@intel.com>
Reviewed-by: Liu Shuo <shuo.a.liu@intel.com>
Acked-by: Yu Wang <yu1.wang@intel.com>

Author: YangLiang3

devicemodel/hw/pci/xhci.c

@@ -4001,13 +4001,13 @@ pci_xhci_init(struct vmctx *ctx, struct pci_vdev *dev, char *opts)
 	 * ended by EXCAP_GROUP_END at last item.
 	 */
 	excap = xdev->excap_ptr;
-	xdev->excapoff = excap->start;
-
 	if (!excap) {
-		UPRINTF(LWRN, "Failed to set xHCI extended capability\r\n");
-		return -1;
+		error = -1;
+		goto done;
 	}
 
+	xdev->excapoff = excap->start;
+
 	do {
 		xdev->regsend = excap->end;
 		excap++;