partr: fix rare segfault (#32530)

The write to 'prio' might be arbitrarily delayed, after some other
thread has already popped the task. This commit ensures that all writes
to that field are always behind the same lock as the modification to the
queue itself.

fix #32520

Author: vtjnash

src/partr.c

@@ -123,10 +123,10 @@ static inline int multiq_insert(jl_task_t *task, int16_t priority)
 
     heaps[rn].tasks[heaps[rn].ntasks++] = task;
     sift_up(&heaps[rn], heaps[rn].ntasks-1);
-    jl_mutex_unlock_nogc(&heaps[rn].lock);
     int16_t prio = jl_atomic_load(&heaps[rn].prio);
     if (task->prio < prio)
-        jl_atomic_compare_exchange(&heaps[rn].prio, prio, task->prio);
+        jl_atomic_store(&heaps[rn].prio, task->prio);
+    jl_mutex_unlock_nogc(&heaps[rn].lock);
 
     return 0;
 }