BUG#47598 - MyISAM may write uninitialized data to disk

When MyISAM writes newly created index page it may be
initialized partially. In other words some bytes of
sensible data and uninitialized tail of the page may
go into index file.

Under certain rare circumstances these hunks of memory
may contain data that would be otherwise inaccessible
to user, like passwords or data from other tables.

Fixed by initializing memory for temporary MyISAM key
buffer to '\0'.

No test case for this fix as it is heavily covered by
existing tests.

storage/myisam/mi_open.c:
  When creating new MI_INFO object, initialize MI_INFO::buff.
  This is done to ensure that we never write uninitialized
  memory hunks to index file.
storage/myisam/mi_page.c:
  No need to silence memory error detector anymore,
  page buffer is always initialized.
storage/myisam/mi_write.c:
  Fixed invalid memory read of 2 bytes. new_right_length
  is length of data on a page, including first 2 bytes
  that store this length itself. pos + k_length is pure
  data excluding these 2 bytes containing length.
storage/myisam/rt_index.c:
  To avoid uninitialized data write, create new page
  on info->buff, instead of locally allocated buffer.
  
  Note: second key block on info->buff is used here,
  because first block is used by called functions.
storage/myisam/rt_split.c:
  To avoid uninitialized data write, create new page
  on info->buff, instead of locally allocated buffer.

storage/myisam/mi_open.c

@@ -652,6 +652,9 @@ MI_INFO *mi_open(const char *name, int mode, uint open_flags)
   myisam_open_list=list_add(myisam_open_list,&m_info->open_list);
 
   pthread_mutex_unlock(&THR_LOCK_myisam);
+
+  bzero(info.buff, share->base.max_key_block_length * 2);
+
   if (myisam_log_file >= 0)
   {
     intern_filename(name_buff,share->index_file_name);