compose: Fix iterator for empty tables

The current `xkb_compose_table_iterator_next` segfaults when used with an
empty table. Indeed, in this case we initialize cursors in
`xkb_compose_table_iterator_new` with the dummy node and the direction
`NODE_LEFT`, but the dummy node is a leaf!

Fixed by initializing with no cursors when the table is has no non-dummy
nodes.

src/compose/table.c

@@ -275,7 +275,6 @@ XKB_EXPORT struct xkb_compose_table_iterator *
 xkb_compose_table_iterator_new(struct xkb_compose_table *table)
 {
     struct xkb_compose_table_iterator *iter;
-    struct xkb_compose_table_iterator_cursor cursor;
     xkb_keysym_t *sequence;
 
     iter = calloc(1, sizeof(*iter));
@@ -292,10 +291,15 @@ xkb_compose_table_iterator_new(struct xkb_compose_table *table)
     iter->entry.sequence_length = 0;
 
     darray_init(iter->cursors);
-    cursor.direction = NODE_LEFT;
-    /* Offset 0 is a dummy null entry, skip it. */
-    cursor.node_offset = 1;
-    darray_append(iter->cursors, cursor);
+    /* Add first cursor only if there is at least one non-dummy node */
+    if (darray_size(iter->table->nodes) > 1) {
+        const struct xkb_compose_table_iterator_cursor cursor = {
+            .direction = NODE_LEFT,
+            /* Offset 0 is a dummy null entry, skip it. */
+            .node_offset = 1
+        };
+        darray_append(iter->cursors, cursor);
+    }
 
     return iter;
 }

test/compose.c

@@ -708,7 +708,17 @@ static void
 test_traverse(struct xkb_context *ctx)
 {
     struct xkb_compose_table *table;
+    struct xkb_compose_table_iterator *iter;
 
+    /* Empty table */
+    table = xkb_compose_table_new_from_buffer(ctx, "", 0, "",
+                                              XKB_COMPOSE_FORMAT_TEXT_V1,
+                                              XKB_COMPOSE_COMPILE_NO_FLAGS);
+    assert(table);
+    iter = xkb_compose_table_iterator_new(table);
+    assert (xkb_compose_table_iterator_next(iter) == NULL);
+
+    /* Non-empty table */
     const char *buffer = "<dead_circumflex> <dead_circumflex> : \"foo\" X\n"
                          "<Ahook> <x> : \"foobar\"\n"
                          "<Multi_key> <o> <e> : oe\n"
@@ -725,7 +735,7 @@ test_traverse(struct xkb_context *ctx)
                                               XKB_COMPOSE_COMPILE_NO_FLAGS);
     assert(table);
 
-    struct xkb_compose_table_iterator *iter = xkb_compose_table_iterator_new(table);
+    iter = xkb_compose_table_iterator_new(table);
 
     test_eq_entry(xkb_compose_table_iterator_next(iter),
                   XKB_KEY_eacute, "é",