MDEV-21490: binlog tests fail with valgrind: Conditional jump or move depends on uninitialised value in sql_ex_info::init

Problem:
=======
P1) Conditional jump or move depends on uninitialised value(s)
    sql_ex_info::init(char const*, char const*, bool) (log_event.cc:3083)

code: All the following variables are not initialized.
----
  return ((cached_new_format != -1) ? cached_new_format :
    (cached_new_format=(field_term_len > 1 || enclosed_len > 1 ||
    line_term_len > 1 || line_start_len > 1 || escaped_len > 1)));

P2) Conditional jump or move depends on uninitialised value(s)
    Rows_log_event::Rows_log_event(char const*, unsigned
      int, Format_description_log_event const*) (log_event.cc:9571)

Code: Uninitialized values is reported for 'var_header_len' variable.
----
  if (var_header_len < 2 || event_len < static_cast<unsigned
      int>(var_header_len + (post_start - buf)))

P3) Conditional jump or move depends on uninitialised value(s)
    Table_map_log_event::pack_info(Protocol*) (log_event.cc:11553)

code:'m_table_id' is uninitialized.
----
  void Table_map_log_event::pack_info(Protocol *protocol)
  ...
  size_t bytes= my_snprintf(buf, sizeof(buf), "table_id: %lu (%s.%s)",
                              m_table_id, m_dbnam, m_tblnam);

Fix:
===
P1 - Fix)
Initialize cached_new_format,field_term_len, enclosed_len, line_term_len,
line_start_len, escaped_len members in default constructor.

P2 - Fix)
"var_header_len" is initialized by reading the event buffer. In case of an
invalid event the buffer will contain invalid data. Hence added a check to
validate the event data. If event_len is smaller than valid header length
return immediately.

P3 - Fix)
'm_table_id' within Table_map_log_event is initialized by reading data from
the event buffer. Use 'VALIDATE_BYTES_READ' macro to validate the current
state of the buffer. If it is invalid return immediately.