dnsdist: Appease clang++ 12 asan on MacOS

[omoerbeek]

MacOS clang++ 12 with asan does not like to access a vector of bytes
outside it's begin()..end(), even thouh we assured there is capacity.
So make sure the size is set before groping inside the vector.

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