[bugfix](becore) use after free problem when the segment is pop (#41685) (#41697)

## Proposed changes

pick #41685
Issue Number: close #xxx
introduced by #41608

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## Proposed changes

Issue Number: close #xxx

<!--Describe your changes.-->

Co-authored-by: yiguolei <yiguolei@gmail.com>

be/src/olap/segment_loader.h

@@ -166,7 +166,7 @@ public:
         if (segments.empty()) {
             return nullptr;
         }
-        auto& last_segment = segments.back();
+        segment_v2::SegmentSharedPtr last_segment = segments.back();
         if (last_segment->healthy_status().ok()) {
             return nullptr;
         }