net: lib: coap: Initialize response_truncated

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

Signed-off-by: Tommi Rantanen <tommi.rantanen@nordicsemi.no>