ACRN:DM: Add pointer check before use the erst
The event ring segment table  pointer may be NULL when get the address
from guest, add pointer check before use it.
Acked-by: Wang, Yu1
@@ -1817,6 +1817,11 @@ pci_xhci_insert_event(struct pci_xhci_vdev *xdev,
int erdp_idx;
rts = &xdev->rtsregs;
+ if (&rts->erstba_p == NULL) {
+ UPRINTF(LFTL, "Invalid Event Ring Segment Table base address!\r\n");
+ return -EINVAL;
+ }
+
erdp = rts->intrreg.erdp & ~0xF;
erst = &rts->erstba_p[rts->er_enq_seg];
erdp_idx = (erdp - erst->qwRingSegBase) / sizeof(struct xhci_trb);
