Change the tree_next to a flexible array member

Signed-off-by: Mikhail Kurnosov <mkurnosov@gmail.com>

ompi/mca/coll/base/coll_base_topo.h

@@ -20,12 +20,13 @@
 #define MCA_COLL_BASE_TOPO_H_HAS_BEEN_INCLUDED
 
 #include "ompi_config.h"
+#include <stddef.h>
 
 #define MAXTREEFANOUT 32
 
-#define MEMBSIZE(type, member) (sizeof(((type *)0)->member))
+#define MEMBSIZE(type, member) (sizeof(((type *)0)->member[0]))
 #define COLL_TREE_SIZE(fanout) \
-        (sizeof(ompi_coll_tree_t) + ((fanout) - 1) * MEMBSIZE(ompi_coll_tree_t, tree_next[1]))
+        (offsetof(ompi_coll_tree_t, tree_next) + (fanout) * MEMBSIZE(ompi_coll_tree_t, tree_next))
 
 BEGIN_C_DECLS
 
@@ -35,7 +36,7 @@ typedef struct ompi_coll_tree_t {
     int32_t tree_bmtree;
     int32_t tree_prev;
     int32_t tree_nextsize;
-    int32_t tree_next[1];
+    int32_t tree_next[];
 } ompi_coll_tree_t;
 
 ompi_coll_tree_t*