devfs: Avoid comparison with an uninitialized var in devfs_fp_check()

devvn_refthread() will initialize *devp only if it succeeds, so check for
success before comparing with fp->f_data.  Other devvn_refthread()
callers are careful to do this.

Reported by:	KMSAN
Reviewed by:	kib
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D30068

sys/fs/devfs/devfs_vnops.c

@@ -120,9 +120,8 @@ static int
 devfs_fp_check(struct file *fp, struct cdev **devp, struct cdevsw **dswp,
     int *ref)
 {
-
 	*dswp = devvn_refthread(fp->f_vnode, devp, ref);
-	if (*devp != fp->f_data) {
+	if (*dswp == NULL || *devp != fp->f_data) {
 		if (*dswp != NULL)
 			dev_relthread(*devp, *ref);
 		return (ENXIO);