WL#6036: Decrease malloc use in: get_lock_data

my_malloc calls made when doing a normal PK lookup SELECT query

get_lock_data
Called from sql_lock.lock_tables.open_and_lock_tables

From Mikael:

get_lock_data
My notes are a bit unclear exactly when the second call comes, but
I think it is in unlock_tables phase. Seems like get_lock_data
allocates some short-time memory both for lock/unlock