fix: run_task/deactivate_task race condition on m_imp->m_closure (#2959)

Fixes #2853, unblocking my work before I get to refactoring this part of
the task manager.

src/runtime/object.cpp

@@ -714,8 +714,12 @@ class task_manager {
             resolve_core(t, v);
         } else {
             // `bind` task has not finished yet, re-add as dependency of nested task
+            // NOTE: closure MUST be extracted before unlocking the mutex as otherwise
+            // another thread could deactivate the task and empty `m_clousure` in
+            // between.
+            object * c = t->m_imp->m_closure;
             lock.unlock();
-            add_dep(lean_to_task(closure_arg_cptr(t->m_imp->m_closure)[0]), t);
+            add_dep(lean_to_task(closure_arg_cptr(c)[0]), t);
             lock.lock();
         }
     }