MDEV-7526: TokuDB doesn't build on OS X

Fixed compile warning related to if statement always being true. The if
statement can not be false, as the address of a member field is always
true.

storage/tokudb/ft-index/ft/txn/rollback.cc

@@ -147,9 +147,7 @@ static inline PAIR_ATTR make_rollback_pair_attr(long size) {
 PAIR_ATTR
 rollback_memory_size(ROLLBACK_LOG_NODE log) {
     size_t size = sizeof(*log);
-    if (&log->rollentry_arena) {
-        size += log->rollentry_arena.total_footprint();
-    }
+    size += log->rollentry_arena.total_footprint();
     return make_rollback_pair_attr(size);
 }