Fixes to avoid reading message fields out of bounds

[Aleksander Morgado]

Hey all!

I've pushed several commits to a branch to try to make sure we never
try to read message fields out of the bounds of the message, e.g. for
malformed messages or when a "Failure" command_done message comes
without any field (e.g.
https://gitlab.freedesktop.org/mobile-broadband/ModemManager/issues/161)

Please see https://gitlab.freedesktop.org/mobile-broadband/libmbim/merge_requests/18

Comments welcome!

-- 
Aleksander
https://aleksander.es