fix: npe because of clone is not thread safe in BreakpointInfo

[rantianhua]

Closes #143

[codecov-io]

Codecov Report

Merging #323 into master will decrease coverage by 0.06%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master     #323      +/-   ##
==========================================
- Coverage   83.73%   83.66%   -0.07%     
==========================================
  Files          74       74              
  Lines        4149     4145       -4     
  Branches      666      669       +3     
==========================================
- Hits         3474     3468       -6     
+ Misses        403      402       -1     
- Partials      272      275       +3

Impacted Files	Coverage Δ
...huo/okdownload/core/breakpoint/BreakpointInfo.java	75.51% <60%> (-3.88%)	⬇️
...ulishuo.okdownload.kotlin/DownloadTaskExtension.kt	86.45% <0%> (+0.82%)	⬆️

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