Return early in printreg() when the pointer is NULL. For FreeBSD, the

strings describing the drive status and error bits are so deficient
that the pointer is always NULL.

Reported by:	Philippe Charnier <charnier@lirmm.fr>

usr.bin/mt/mt.c

@@ -302,6 +302,8 @@ printreg(s, v, bits)
 		printf("%s=%o", s, v);
 	else
 		printf("%s=%x", s, v);
+	if (!bits)
+		return;
 	bits++;
 	if (v && bits) {
 		putchar('<');