BUG#51426: overflow for auto_increment column causes slave to stop

In BUG#49562 we fixed the case where numeric user var events
would not serialize the flag stating whether the value was signed
or unsigned (unsigned_flag). This fixed the case that the slave
would get an overflow while treating the unsigned values as
signed.
      
In this bug, we find that the unsigned_flag can sometimes change
between the moment that the user value is recorded for binlogging
purposes and the actual binlogging time. Since we take the
unsigned_flag from the runtime variable data, at binlogging time,
and the variable value is comes from the copy taken earlier in
the execution, there may be inconsistency in the
User_var_log_event between the variable value and its
unsigned_flag.
      
We fix this by also copying the unsigned_flag of the
user_var_entry when its value is copied, for binlogging
purposes. Later, at binlogging time, we use the copied
unsigned_flag and not the one in the runtime user_var_entry
instance.

sql/sql_class.h

@@ -75,6 +75,7 @@ typedef struct st_user_var_events
   ulong length;
   Item_result type;
   uint charset_number;
+  bool unsigned_flag;
 } BINLOG_USER_VAR_EVENT;
 
 #define RP_LOCK_LOG_IS_ALREADY_LOCKED 1