igraph dfs may signal unreachable nodes with 0

[jefferis]

• this still seems to be up in the air but at least some preparatory branches for igraph 2.0 use a 0 rather than NaN to signal unreachable nodes
• see https://github.com/natverse/nat/issues/517

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (c608307) 76.88% compared to head (ee4a57b) 76.89%. Report is 2 commits behind head on master.

Files	Patch %	Lines
R/ngraph.R	85.71%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #518 +/- ## ======================================= Coverage 76.88% 76.89% ======================================= Files 48 48 Lines 5935 5937 +2 ======================================= + Hits 4563 4565 +2 Misses 1372 1372 ```

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