network_monitor: Fix potential NULL dereference.

[ossama-othman]

Correct a potential NULL pointer dereference when cleaning up resources in the case of a failed attempt to track a network address.

Fixes #230.

[coveralls]

Pull Request Test Coverage Report for Build 3039261346

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.

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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 64.689%

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Totals
Change from base Build 2893798308:	0.0%
Covered Lines:	1385
Relevant Lines:	2141

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💛 - Coveralls

[mjmartineau]

Looks good.