V4L/DVB (7306): pvrusb2: Fix oops possible when claiming a NULL stream

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>

drivers/media/video/pvrusb2/pvrusb2-context.c

@@ -173,7 +173,7 @@ int pvr2_channel_claim_stream(struct pvr2_channel *cp,
 	int code = 0;
 	pvr2_context_enter(cp->mc_head); do {
 		if (sp == cp->stream) break;
-		if (sp->user) {
+		if (sp && sp->user) {
 			code = -EBUSY;
 			break;
 		}