rc: ensure that vfs.mounts.nfs check works without NFS

If NFS isn't compiled into the kernel, sysctl -n vfs.mounts.nfs
will produce no numerical output.  Make sure that we always have
a valid arithmetic expression.

Reported by and ok patrick@

diff --git a/etc/rc b/etc/rc
index 94465ad..cd1d026 100644 (file)
--- a/etc/rc
+++ b/etc/rc
@@ -1,4 +1,4 @@
-#      $OpenBSD: rc,v 1.544 2020/11/29 20:14:06 deraadt Exp $
+#      $OpenBSD: rc,v 1.545 2021/02/09 21:42:04 naddy Exp $
 
 # System startup script run by init on autoboot or after single-user.
 # Output and error are redirected to console by init, and the console is the
@@ -424,7 +424,7 @@ if [[ $pf != NO ]]; then
        pass in proto carp keep state (no-sync)
        pass out proto carp !received-on any keep state (no-sync)"
 
-       if (($(sysctl -n vfs.mounts.nfs 2>/dev/null) > 0)); then
+       if (($(sysctl -n vfs.mounts.nfs 2>/dev/null)+0 > 0)); then
                # Don't kill NFS.
                RULES="set reassemble yes no-df
                $RULES