Fix vtrnd pci_vtrnd_notify uninitialized memory use (GHSL-2021-056)

Signed-off-by: Frederic Dalleau <frederic.dalleau@docker.com>

Author: fredericdalleau

src/lib/pci_virtio_rnd.c

@@ -100,7 +100,7 @@ pci_vtrnd_notify(void *vsc, struct vqueue_info *vq)
 {
 	struct iovec iov;
 	struct pci_vtrnd_softc *sc;
-	int len;
+	int len, n;
 	uint16_t idx;
 
 	sc = vsc;
@@ -111,7 +111,11 @@ pci_vtrnd_notify(void *vsc, struct vqueue_info *vq)
 	}
 
 	while (vq_has_descs(vq)) {
-		vq_getchain(vq, &idx, &iov, 1, NULL);
+		n = vq_getchain(vq, &idx, &iov, 1, NULL);
+		if (n < 0) {
+			fprintf(stderr, "vtrnd: vtrnd_notify(): n %d\r\n", n);
+			return;
+		}
 
 		len = (int) read(sc->vrsc_fd, iov.iov_base, iov.iov_len);