mq_tree_size(), mq_tree_position(): Fix conditional expressions

Fixes:

      CC       src/marquee-notebook.o
    In file included from src/notebook.c:29:0:
    src/notebook.c: In function ‘mq_notebook_get_n_pages’:
    src/tree.h:67:70: warning: pointer/integer type mismatch in conditional expression
     #define mq_tree_size(node)     ((node) ? (((MqTree *) (node))->size) : NULL)
                                                                          ^
    src/notebook.c:253:9: note: in expansion of macro ‘mq_tree_size’
      return mq_tree_size(MQ_TREE(notebook->tree));
             ^
    src/notebook.c:253:2: warning: return makes integer from pointer without a cast
      return mq_tree_size(MQ_TREE(notebook->tree));
      ^

1 files changed, 2 insertions, 2 deletions

diff --git a/src/tree.h b/src/tree.h
index 412ebfe..3d2c113 100644
--- a/src/tree.h
+++ b/src/tree.h
@@ -64,8 +64,8 @@ mq_tree_insert_sibling_allocated(MqTree *node, MqTree *sibling, gpointer data);
 void
 mq_tree_remove(MqTree *node);
 
-#define mq_tree_size(node)     ((node) ? (((MqTree *) (node))->size) : NULL)
-#define mq_tree_position(node) ((node) ? (((MqTree *) (node))->position) : NULL)
+#define mq_tree_size(node)     ((node) ? (((MqTree *) (node))->size) : -1)
+#define mq_tree_position(node) ((node) ? (((MqTree *) (node))->position) : -1)
 #define mq_tree_root(node)     ((node) ? (((MqTree *) (node))->root) : NULL)
 #define mq_tree_previous(node) ((node) ? (((MqTree *) (node))->prev) : NULL)
 #define mq_tree_next(node)     ((node) ? (((MqTree *) (node))->prev) : NULL)