sock_dns: correctly report too short messages

[miri64]

Contribution description

While writing the tests I promised in #10740 I noticed this bug. sock_dns_query() returns the length of a too short DNS response instead of just reporting that it is wrong. While this is not severe, it can lead to confusing behavior, as the function does not report an error but just a number >0. This basically means, sock_dns_query() will tell the user that the address is whatever currently is on the stack ;-).

Testing procedure

After the interface configuration described in the README of tests/gnrc_sock_dns start the application (but not the dnsmasq) and the following command in a separate terminal

while sleep 1; do echo AACBAAAACpoAAAAA | base64 -d | sudo socat fd:0 udp6-sendto:'[<native-lladdr>%tap0]':49152,bind="[2001:db8::1]:53",sourceport=53; done

Without this PR the response the output will be

example.org resolves to ::

With it correctly reports

error resolving example.org

Issues/PRs references

None, but found while working on https://github.com/miri64/RIOT/tree/tests/enh/gnrc_sock_dns/tests/gnrc_sock_dns

[kaspar030]

LGTM, pls squash!

[kaspar030]

(untested) ACK.

[miri64]

Squashed

[miri64]

Backport provided in #10900