NetmaskTree: do not test node for null, the loop guarantees node is not null.

[omoerbeek]

Found by Coverity (1419400), which concludes that if node could be
null, the following node->node.first.getBits() would deref a nullptr.

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I have:

• read the CONTRIBUTING.md document
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• tested this code
• included documentation (including possible behaviour changes)
• documented the code
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[rgacogne]

I agree that node can't be nullptr with the current code, either when first entering the loop or while iterating, so I guess that leaving that check provides a false feeling of safety.