Add assertions to check that we do not go out of bounds of io thread status array
os0file.c:
  Fix memory corruption (assertion failure on line 244 of sync0sync.c) reported by Miguel in a Windows build of MySQL-4.1.2. The bug is present in all InnoDB versions in Windows, but it depends on how the linker places a static array in srv0srv.c, whether the bug shows itself. 4 bytes were overwritten with a pointer to a statically allocated string: 'get windows aio return value'.