sock_dns: correctly report too short messages [backport 2019.01]

[miri64]

Backport of #10896

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

[aabadie]

is it normal that we have a big green merge button without having a reviewer ACK ? My guess is that a configuration is missing for the 2019.01-branch protection.

[miri64]

is it normal that we have a big green merge button without having a reviewer ACK ? My guess is that a configuration is missing for the 2019.01-branch protection.

You are the release manager. Did you follow https://github.com/RIOT-OS/RIOT/wiki/%5Bdraft%5D-Managing-a-Release. It told you to do it ;-).

[miri64]

You are the release manager. Did you follow https://github.com/RIOT-OS/RIOT/wiki/%5Bdraft%5D-Managing-a-Release. It told you to do it ;-).

It might be, that you need special permission for that though :-/. Maybe that's a bug in the script that it does not warn you, if you used that.

[miri64]

Just noticed that GitHub now also allows to set branch-protection based on a pattern that matches the branch-name. I added the rule for the [0-9][0-9][0-9][0-9].[0-9][0-9]-branch pattern ;-).

[aabadie]

Maybe that's a bug in the script that it does not warn you, if you used that.

I'm a lazy guy so I used that very useful script :)
I think only administrators of the project can change this kind of settings.

I added the rule for the [0-9][0-9][0-9][0-9].[0-9][0-9]-branch pattern

Thanks !

[aabadie]

Changes are the same as in original PR.

ACK and go.