[Fix] access deleted point in prefetch thread.

[BigVan]

What this PR does / why we need it:

The DynamicPrefetcher::replay(IFile* imgfile) progress is running in a background thread and it will access an invalid pointer after *imgfile has been deleted in stack_files()

overlaybd/src/overlaybd/lsmt/file.cpp

Line 1790 in 7093da0

delete l;

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?