MDEV-6582: DEBUG_SYNC does not reset mysys_var->current_mutex, causes assertion "Trying to unlock mutex that wasn't locked"

The bug was in DEBUG_SYNC. When waiting, debug_sync_execute() temporarily sets
thd->mysys_var->current_mutex to a new value while waiting. However, if the
old value of current_mutex was NULL, it was not restored, current_mutex
remained set to the temporary value (debug_sync_global.ds_mutex).

This made possible the following race: Thread T1 goes to KILL thread T2. In
THD::awake(), T1 loads T2->mysys_var->current_mutex, it is set to ds_mutex, T1
locks this mutex.

Now T2 runs, it does ENTER_COND, it sets T2->mysys_var->current_mutex to
LOCK_wait_commit (for example).

Then T1 resumes, it reloads mysys_var->current_mutex, now it is set to
LOCK_wait_commit, T1 unlocks this mutex instead of the ds_mutex that it locked
previously.

This causes safe_mutex to assert with the message: "Trying to unlock mutex
LOCK_wait_commit that wasn't locked".

The fix is to ensure that DEBUG_SYNC also will restore
mysys_var->current_mutex in the case where the original value was NULL.