coap: Fix zero-length options

This error prevented me from connecting to leshan.eclipseprojects.io
using the DTLS enabled client binary.

[eclipse-wakaamaGH-594] examples: client: dtls: Fix crash

This seems to be the easiest fix that also works for the non-DTLS
client binary.

Tested by successfully connecting to leshan.eclipseprojects.io

ci: Build all examples as standalone

This ensures the examples can be built independently as described in the
readme file.

ci: Build all examples as standalone

This ensures the examples can be built independently as described in the
readme file.

This is loosely related to [eclipse-wakaamaGH-588].

ci: Build all examples as standalone

This ensures the examples can be built independently as described in the
readme file.

This is losely related to [eclipse-wakaamaGH-588].

Silence -Wfloat-equal

Quick and dirty workaround to get rid of this warning.

coap: Detect reserved option delta/length values

RFC 7252 on Option Delta of 15:
> Reserved for the Payload Marker. If the field is set to this value but
> the entire byte is not the payload marker, this MUST be processed as a
> message format error.

RFC 7252 on Option Length of 15:
> Reserved for future use. If the field is set to this value, it MUST be
> processed as a message format error.

ci: Add fuzzing target for coap_parse_message()

This is a first shot at fuzzing in this project.

I picked coap_parse_message() as a starting point because of its
increased exposedness by interpreting unsanitized data coming directly
form the internet.

Once we get the grip on using this tool we might want to integrate it
into the CI pipeline.

coap: Detect reserved option delta/length values

RFC 7252 on Option Delta of 15:
> Reserved for the Payload Marker. If the field is set to this value but
> the entire byte is not the payload marker, this MUST be processed as a
> message format error.

RFC 7252 on Option Length of 15:
> Reserved for future use. If the field is set to this value, it MUST be
> processed as a message format error.

coap: Detect reserved option delta/length values

RFC 7252 on Option Delta of 15:
> Reserved for the Payload Marker. If the field is set to this value but
> the entire byte is not the payload marker, this MUST be processed as a
> message format error.

RFC 7252 on Option Length of 15:
> Reserved for future use. If the field is set to this value, it MUST be
> processed as a message format error.