net: lib: coap: Initialize response_truncated

[trantanen]

Fix the following compilation warning given when using newlibc:
warning: 'response_truncated' may be used uninitialized [-Wmaybe-uninitialized]

Issue is not seen with picolibc.
The variable was introduced as part of PR #76257

[github-actions]

Hello @trantanen, and thank you very much for your first pull request to the Zephyr project!
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[trantanen]

FYI: @mrodgers-witekio @SeppoTakalo

[github-actions]

Hi @trantanen!
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