common/hostlist: fix out of bounds array access

Problem: Internally within the hostlist data structure, an
array of hostranges is kept.  An out of bounds array access
can occur if the number of hostranges is equal to the length
of the array.

Add check to ensure out of bound array access is not allowed
when iterating hosts.

src/common/hostlist.c

@@ -2278,6 +2278,11 @@ static void _iterator_advance(hostlist_iterator_t i)
         return;
     if (++(i->depth) > (i->hr->hi - i->hr->lo)) {
         i->depth = 0;
+        if (i->idx >= (i->hl->size - 1)) {
+            ++i->idx;
+            i->hr = NULL;
+            return;
+        }
         i->hr = i->hl->hr[++i->idx];
     }
 }