bug fix dfget client timeout set error when fileLength is 0

[ww3122000]

Ⅰ. Describe what this PR did

bug fix dfget client timeout set error when fileLength is 0

Ⅱ. Does this pull request fix one issue?

bug fix dfget client timeout set error when fileLength is 0

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

Ⅳ. Describe how to verify it

use dfget and set timeout

Ⅴ. Special notes for reviews

[pouchrobot]

We found this is your first time to contribute to Dragonfly, @ww3122000
👏 We really appreciate it.
Just remind that you have read the contribution guide: https://github.com/dragonflyoss/Dragonfly/blob/master/CONTRIBUTING.md
If you didn't, you should do that first. If done, welcome again and please enjoy hacking! 🍻

[allencloud]

Thanks for your contribution, Please help to sign https://github.com/dragonflyoss/Dragonfly/pull/1032/checks?check_run_id=277021510. @ww3122000

[codecov-io]

Codecov Report

Merging #1032 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1032      +/-   ##
==========================================
- Coverage   46.64%   46.62%   -0.02%     
==========================================
  Files         115      115              
  Lines        7002     7002              
==========================================
- Hits         3266     3265       -1     
  Misses       3481     3481              
- Partials      255      256       +1

Impacted Files	Coverage Δ
dfget/core/core.go	45.83% <0%> (ø)	⬆️
supernode/daemon/mgr/scheduler/manager.go	21.91% <0%> (-0.69%)	⬇️

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[allencloud]

In addition, if you need to real-time chatting, please see #291 to join us.

[allencloud]

LGTM

[starnop]

LGTM.