[fix](schema scan) Fix invalid pointer access (#48313)

Schema scanner runs on a separate thread which is executed
asynchronously. We should make sure all context used not be freed once
it is scheduled.

ERROR: AddressSanitizer: heap-buffer-overflow on address 0x613002f33eb2
at pc 0x55e085dccbe3 bp 0x7f345c0e1f10 sp 0x7f345c0e1f08
READ of size 1 at 0x613002f33eb2 thread T2776 (FragmentMgrAsyn)
#0 0x55e085dccbe2 in std::__atomic_base::load(std::memory_order) const
/var/local/ldb-toolchain/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/bits/atomic_base.h:481:9
#1 0x55e085dccbe2 in std::atomic::operator bool() const
/var/local/ldb-toolchain/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/atomic:87:22
#2 0x55e085dccbe2 in
doris::SchemaScanner::get_next_block_async(doris::RuntimeState*)::$_0::operator()()
const
/home/zcp/repo_center/doris_master/doris/be/src/exec/schema_scanner.cpp:118:5
#3 0x55e085dccbe2 in void std::__invoke_impl(std::__invoke_other,
doris::SchemaScanner::get_next_block_async(doris::RuntimeState*)::$_0&)
/var/local/ldb-toolchain/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/bits/invoke.h:61:14
#4 0x55e085dccbe2 in std::enable_if, void>::type
std::__invoke_r(doris::SchemaScanner::get_next_block_async(doris::RuntimeState*)::$_0&)
/var/local/ldb-toolchain/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/bits/invoke.h:111:2
#5 0x55e085dccbe2 in std::_Function_handler::_M_invoke(std::_Any_data
const&)
/var/local/ldb-toolchain/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/bits/std_function.h:291:9
#6 0x55e050f081ca in doris::ThreadPool::dispatch_thread()
/home/zcp/repo_center/doris_master/doris/be/src/util/threadpool.cpp:608:24
#7 0x55e050ede467 in doris::Thread::supervise_thread(void*)
/home/zcp/repo_center/doris_master/doris/be/src/util/thread.cpp:498:5
    #8 0x7f376ef5aac2 in start_thread nptl/pthread_create.c:442:8
#9 0x7f376efec84f misc/../sysdeps/unix/sysv/linux/x86_64/clone3.S:81

Author: Gabriel39

be/src/exec/schema_scanner.cpp

@@ -119,12 +119,12 @@ Status SchemaScanner::get_next_block_async(RuntimeState* state) {
     auto task_ctx = state->get_task_execution_context();
     RETURN_IF_ERROR(ExecEnv::GetInstance()->fragment_mgr()->get_thread_pool()->submit_func(
             [this, task_ctx, state]() {
-                DCHECK(_async_thread_running == false);
                 auto task_lock = task_ctx.lock();
                 if (task_lock == nullptr) {
                     _scanner_status.update(Status::InternalError("Task context not exists!"));
                     return;
                 }
+                DCHECK(_async_thread_running == false);
                 SCOPED_ATTACH_TASK(state);
                 _dependency->block();
                 _async_thread_running = true;