Fix for memory overlap in rt_split.c (reported by valgrind)

myisam/rt_split.c:
  Prevent memcpy if source and destination are same

myisam/rt_split.c

@@ -326,6 +326,7 @@ int rtree_split_page(MI_INFO *info, MI_KEYDEF *keyinfo, uchar *page, uchar *key,
       cur2 = rt_PAGE_NEXT_KEY(cur2, key_length, nod_flag);
       ++n2;
     }
+    if (to != cur->key)
       memcpy(to - nod_flag, cur->key - nod_flag, full_length);
   }