Bug #67437 init_io_cache() has suspicious memset()

Description:
init_io_cache() has the following line:

memset(&info->append_buffer_lock, 0, sizeof(info));

So we try to clear info->append_buffer_lock using the size of info,
not the size of info->append_buffer_lock.

Originally reported by Mr. Left on the internals@ mailing list.
See http://lists.mysql.com/internals/38621

How to repeat:
Code inspection.

Suggested fix:
Use the sizeof info->append_buffer_lock.

instead of using sizeof operator with variable / instance type name should be used  because if variable / instance is pointer memset will only set the first 4 bytes, (in context of this issue sizeof(info->append_buffer_lock) will work , but  I will suggest to use sizeof(mysql_mutex_t) for clarity of code). 

my reply to tread: http://lists.mysql.com/internals/38624

Regards,
Shaukat Mahmood Ahmad

Noted in 5.6.9, 5.7.0 changelogs.

init_io_cache() used memset() to clear a mutex but passed the wrong
mutex size.