Reset the connection_name to contain a null string, if the pointer
points to the same space as that of the system variable
default_master_connection.

We do this because the system variable may be updated which could free
the pointer and create a new one, causing use-after-free for
re-execution of prepared statements and stored procedures where the
LEX may be reused.

This allows connection_name to be set again be to the system variable
pointer in the next call of this function (see earlier in this
function), after any possible updates to the system variable.