Problem
=======

When facing decoding of corrupt binary log files, server may misbehave
without detecting the events corruption.

This patch makes MySQL server more resilient to binary log decoding.

Fixes for events de-serialization and apply
===========================================

@sql/log_event.cc

Query_log_event::Query_log_event: added a check to ensure query length
is respecting event buffer limits.

Query_log_event::do_apply_event: extended a debug print, added a check
to character set to determine if it is "parseable" or not, verified if
database name is valid for system collation.

Start_log_event_v3::do_apply_event: report an error on applying a
non-supported binary log version.

Load_log_event::copy_log_event: added a check to table_name length.

User_var_log_event::User_var_log_event: added checks to avoid reading
out of buffer limits.

User_var_log_event::do_apply_event: reported an sanity check error
properly and added individual sanity checks for variable types that
expect fixed (or minimum) amount of bytes to be read.

Rows_log_event::Rows_log_event: added checks to avoid reading out of
buffer limits.

@sql/log_event_old.cc

Old_rows_log_event::Old_rows_log_event: added a sanity check to avoid
reading out of buffer limits.

@sql/sql_priv.h

Added a sanity check to available_buffer() function.