Follow-up to fix issue with access to probably not-initialized mutex/cond_var

Constructor of the class st_debug_sync_globals was changed to initialize
the data members dsp_hits, dsp_executed, dsp_max_active with zero.
Formerly, these data members were filled with zeroes by C-runtime since
the variable debug_sync_global was declared as static and according with C rules
the static variable initialized with zero bytes.

By the same reason, the data members
  debug_sync_global->ds_mutex
  debug_sync_global->ds_cond
were initialized by zeros before the patch for MDEV-31871. After this patch
the memory for the synch primitives debug_sync_global->ds_mutex
and debug_sync_global->ds_cond are initialized explicitly by calling
the functions mysql_mutex_init/mysql_cond_init so access to these synch
primitives should be done only after such initialization be completed.
Guarded access to these synch primitives has been added to the function
debug_sync_end_thread() that is called on clean up since that was single
problem place detected by MSAN. Theoretically problem places located in the
function debug_sync_execute were not protected with similar check since
it is not obvious that the variables debug_sync_global->ds_mutex
and debug_sync_global->ds_cond could be not initilialized for use cases where
the function debug_sync_execute() is called. It is required additional study
to conclude whether it does need or not.