[fix](brpc) coredump caused by brpc checking (apache#44047)

### What problem does this PR solve?

```
/root/doris/be/src/runtime/fragment_mgr.cpp:1064:20: runtime error: member call on null pointer of type 'doris::PBackendService_Stub'
#0 0x55bd899c9aaa in doris::FragmentMgr::_check_brpc_available(std::shared_ptr<doris::PBackendService_Stub> const&, doris::FragmentMgr::BrpcItem const&) /root/doris/be/src/runtime/fragment_mgr.cpp:1064:20
#1 0x55bd899c521f in doris::FragmentMgr::cancel_worker() /root/doris/be/src/runtime/fragment_mgr.cpp:1021:13
#2 0x55bd8a4c97ae in std::function<void ()>::operator()() const /var/local/ldb-toolchain/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/bits/std_function.h:560:9
#3 0x55bd8a4c97ae in doris::Thread::supervise_thread(void*) /root/doris/be/src/util/thread.cpp:498:5
#4 0x7f7601092608 in start_thread /build/glibc-SzIz7B/glibc-2.31/nptl/pthread_create.c:477:8
#5 0x7f760133f132 in __clone /build/glibc-SzIz7B/glibc-2.31/misc/../sysdeps/unix/sysv/linux/x86_64/clone.S:95
   
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /root/doris/be/src/runtime/fragment_mgr.cpp:1064:20 in
*** Query id: 0-0 ***
*** is nereids: 0 ***
*** tablet id: 0 ***
*** Aborted at 1731663847 (unix time) try "date -d @1731663847" if you are using GNU date ***
*** Current BE git commitID: b663df0e50 ***
*** SIGSEGV address not mapped to object (@0x0) received by PID 17169 (TID 17463 OR 0x7f746d21a700) from PID 0; stack trace: ***
0# doris::signal::(anonymous namespace)::FailureSignalHandler(int, siginfo_t*, void*) at /root/doris/be/src/common/signal_handler.h:421
1# PosixSignals::chained_handler(int, siginfo_t*, void*) [clone .part.0] in /usr/lib/jvm/java-17-openjdk-amd64/lib/server/libjvm.so
2# JVM_handle_linux_signal in /usr/lib/jvm/java-17-openjdk-amd64/lib/server/libjvm.so
3# 0x00007F7601263090 in /lib/x86_64-linux-gnu/libc.so.6
4# doris::FragmentMgr::_check_brpc_available(std::shared_ptr<doris::PBackendService_Stub> const&, doris::FragmentMgr::BrpcItem const&) in /mnt/ssd01/pipline/OpenSourceDoris/clusterEnv/P0/Cluster0/be/lib/doris_be
5# doris::FragmentMgr::cancel_worker() at /root/doris/be/src/runtime/fragment_mgr.cpp:1022
6# doris::Thread::supervise_thread(void*) at /root/doris/be/src/util/thread.cpp:499
7# start_thread at /build/glibc-SzIz7B/glibc-2.31/nptl/pthread_create.c:478
8# __clone at ../sysdeps/unix/sysv/linux/x86_64/clone.S:97
```

be/src/runtime/fragment_mgr.cpp

@@ -1017,8 +1017,14 @@ void FragmentMgr::cancel_worker() {
             }
         }
 
-        for (auto it : brpc_stub_with_queries) {
-            _check_brpc_available(it.first, it.second);
+        if (config::enable_brpc_connection_check) {
+            for (auto it : brpc_stub_with_queries) {
+                if (!it.first) {
+                    LOG(WARNING) << "brpc stub is nullptr, skip it.";
+                    continue;
+                }
+                _check_brpc_available(it.first, it.second);
+            }
         }
 
         if (!queries_lost_coordinator.empty()) {

be/src/vec/sink/vdata_stream_sender.cpp

@@ -70,19 +70,13 @@ Status Channel::init(RuntimeState* state) {
         return Status::OK();
     }
 
+    auto network_address = _brpc_dest_addr;
     if (_brpc_dest_addr.hostname == BackendOptions::get_localhost()) {
         _brpc_stub = state->exec_env()->brpc_internal_client_cache()->get_client(
                 "127.0.0.1", _brpc_dest_addr.port);
-        auto network_address = _brpc_dest_addr;
         network_address.hostname = "127.0.0.1";
-        if (config::enable_brpc_connection_check) {
-            state->get_query_ctx()->add_using_brpc_stub(network_address, _brpc_stub);
-        }
     } else {
         _brpc_stub = state->exec_env()->brpc_internal_client_cache()->get_client(_brpc_dest_addr);
-        if (config::enable_brpc_connection_check) {
-            state->get_query_ctx()->add_using_brpc_stub(_brpc_dest_addr, _brpc_stub);
-        }
     }
 
     if (!_brpc_stub) {
@@ -91,6 +85,10 @@ Status Channel::init(RuntimeState* state) {
         LOG(WARNING) << msg;
         return Status::InternalError(msg);
     }
+
+    if (config::enable_brpc_connection_check) {
+        state->get_query_ctx()->add_using_brpc_stub(_brpc_dest_addr, _brpc_stub);
+    }
     return Status::OK();
 }