idxthread: Use len when searching authline, not ASCII NUL

authline is a stralloc string, there might not be a NUL byte terminator
ezmlm-archive: Fix corruption of output files caused by improper termination

Several places in ezmlm-archive wrote out strings with the assumption
they would be ASCII NUL terminated (as C strings) but were in fact
copied from stralloc data with no trailing NUL. Instead, use the
explicit length already recorded with each one.