When RFC 8974 support (module `nanocoap_token_ext`) is in use, the
request token may be longer than the buffer in the separate response
context is large. This adds a check to not overflow the buffer.
Sadly, this is an API change: Preparing the separate response context
can actually fail, so we need to report this with a return value.
The example application has been adapted to only proceed if the separate
reply context could have been prepared, and rather directly emit a
reset message if the token exceeds the static buffer.
Co-authored-by: benpicco <benpicco@googlemail.com>
