THD::proc_info is vulnerable to data race

The description of THD::proc_info in sql_class.h reads:

This member is accessed and assigned without any synchronization.
Therefore, it may point only to constant (statically allocated) strings, which memory won't go away over time.

This old assumption is insufficient: Without synchronization, accessing a variable while another thread writes to it is not guaranteed to be conflict-free.

The THD::wsrep_info of Galera is in a worse condition for beïng a buffer compared to a pointer, where reading the memory mid-write could result in an amalgamated message.