fdhold can return NULL, so add the one remaining missing check for this

condition.

Reviewed by:    attilio
MFC after:      1 week

sys/kern/kern_descrip.c

@@ -2583,6 +2583,8 @@ sysctl_kern_proc_filedesc(SYSCTL_HANDLER_ARGS)
 		return (error);
 	}
 	fdp = fdhold(p);
+	if (fdp == NULL)
+		return (0);
 	PROC_UNLOCK(p);
 	kif = malloc(sizeof(*kif), M_TEMP, M_WAITOK);
 	FILEDESC_SLOCK(fdp);