Add check for old being NULL in utmp_setid(); from Steven McDonald

src/utmp.c

@@ -96,11 +96,13 @@ utmp_setid(sudo_utmp_t *old, sudo_utmp_t *new)
     size_t idlen;
 
     /* Skip over "tty" in the id if old entry did too. */
+    if (old != NULL) {
 	if (strncmp(line, "tty", 3) == 0) {
 	    idlen = MIN(sizeof(old->ut_id), 3);
 	    if (strncmp(old->ut_id, "tty", idlen) != 0)
 		line += 3;
 	}
+    }
     
     /* Store as much as will fit, skipping parts of the beginning as needed. */
     idlen = strlen(line);