[Noticed in the OpenBSD port.]

$ printf "0\n0\n0\n" | ministat - -
Segmentation fault (core dumped)

The problem is [in ministat.c] at line 646:
		nds = argc;
		for (i = 0; i < nds; i++) {
			setfilenames[i] = argv[i];
			if (!strcmp(argv[i], "-"))
---->				setfiles[0] = stdin;
			else
				setfiles[i] = fopen(argv[i], "r");
			if (setfiles[i] == NULL)
				err(2, "Cannot open %s", argv[i]);
		}

The simplest fix is just to change that to:
				setfiles[i] = stdin;

With that in place, ministat will error out reporting that
(on second reading) the input has fewer than 3 data points.