fix a pointer error when dlist pops up the front element

[wjiec]

1. fix the typo in the dlist
2. use a uniform way to determine if dlist is empty
3. fix a pointer error when dlist pops up the front element

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[coveralls]

Pull Request Test Coverage Report for Build 3922599142

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 3890411830:	0.0%
Covered Lines:	1779
Relevant Lines:	1779

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💛 - Coveralls