Add __attribute__((nonnull) hints to omemo_receive_message and
omemo_on_message_recv declarations to catch NULL arguments at
compiletime.
So we can set the error state unconditionally in the functions.
If an incoming OMEMO message failed to decrypt (due to missing session
keys or untrusted identities), Profanity would fall back to displaying
the raw XMPP body. This usually contained a generic string like "This
message is encrypted with OMEMO,".
We wrote the detailed reason in the debug logs but the user only saw the
fallback message and probably wondered *why* the message wasn't
displayed properly.
I'm unsure if we should display the fallback message as well though.
We try to decrypt all messages, if it's successful we use
sv_ev_incoming_message even for OMEMO messages. We pass an OMEMO
boolean to let UI be aware that message were encrypted.
