as some compilers don't handle that

This was introduced when I was experimenting with support for logging progress
in Olm. That is now relegated to the logging_enabled branch, so this should
probably be removed.

This also fixes the incompatibility of session pickles from the current master
branch with those from olm 0.1.0.

Initialise the length fields of the reader struct in decode_message, even if the message is invalid, fixes a crash where the message was too short