librlist: ignore missing ranks in rlist_mark_up/down

Problem: rlist_mark_state() doesn't check the return value from
rlist_find_rank(). If a rank is passed to rlist_mark_up() or
rlist_mark_down() that doesn't exist in the current rlist, then this
will result in a dereference of NULL and a segfault.

Skip ranks that don't exist in rlist_mark_start(), effectively
ignoring invalid ranks passed to rlist_mark_up() and rlist_mark_down().

This is simpler than returning an error, which would require multiple
iterations of the ranks idset as well as more complex error handling
on the caller side.

Author: grondo

src/common/librlist/rlist.c

@@ -2313,9 +2313,11 @@ static int rlist_mark_state (struct rlist *rl, bool up, const char *ids)
     i = idset_first (idset);
     while (i != IDSET_INVALID_ID) {
         struct rnode *n = rlist_find_rank (rl, i);
-        if (n->up != up)
-            count += idset_count (n->cores->avail);
-        n->up = up;
+        if (n) {
+            if (n->up != up)
+                count += idset_count (n->cores->avail);
+            n->up = up;
+        }
         i = idset_next (idset, i);
     }
     idset_destroy (idset);