staging: vc04_services: fix NULL pointer dereference on pointer 'service'

Currently, if pservice is null then service is set to NULL and immediately
afterwards service is dereferenced causing a null pointer dereference. Fix
this by bailing out early of the function with a null return.

Detected by CoverityScan, CID#1419681 ("Explicit null dereferenced")
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Acked-by: NStefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vc04_services/interface/vchiq_arm/vchiq_core.c

@@ -2673,7 +2673,7 @@ vchiq_add_service_internal(VCHIQ_STATE_T *state,
 
 	if (!pservice) {
 		kfree(service);
-		service = NULL;
+		return NULL;
 	}
 
 	service_quota = &state->service_quotas[service->localport];