xhci-mem.c: Check for ring->first_seg != NULL

There are 2 situations wherein the xhci_ring* might not get freed:
- When xhci_ring_alloc() -> xhci_segment_alloc() returns NULL and
  we goto the fail: label in xhci_ring_alloc. In this case, the ring
  will not get kfreed.
- When the num_segs argument to xhci_ring_alloc is passed as 0 and
  we try to free the rung after that.
  ( This doesn't really happen as of now in the code but we seem to
    be entertaining num_segs=0 in xhci_ring_alloc )

This should be backported to kernels as old as 2.6.31.
Signed-off-by: Kautuk Consul <consul.kautuk@gmail.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/host/xhci-mem.c

@@ -112,18 +112,20 @@ void xhci_ring_free(struct xhci_hcd *xhci, struct xhci_ring *ring)
 	struct xhci_segment *seg;
 	struct xhci_segment *first_seg;
 
-	if (!ring || !ring->first_seg)
+	if (!ring)
 		return;
-	first_seg = ring->first_seg;
-	seg = first_seg->next;
-	xhci_dbg(xhci, "Freeing ring at %p\n", ring);
-	while (seg != first_seg) {
-		struct xhci_segment *next = seg->next;
-		xhci_segment_free(xhci, seg);
-		seg = next;
+	if (ring->first_seg) {
+		first_seg = ring->first_seg;
+		seg = first_seg->next;
+		xhci_dbg(xhci, "Freeing ring at %p\n", ring);
+		while (seg != first_seg) {
+			struct xhci_segment *next = seg->next;
+			xhci_segment_free(xhci, seg);
+			seg = next;
+		}
+		xhci_segment_free(xhci, first_seg);
+		ring->first_seg = NULL;
 	}
-	xhci_segment_free(xhci, first_seg);
-	ring->first_seg = NULL;
 	kfree(ring);
 }