I have some folders with a good amount of spam. While demonstrating full text search to a user, we found a folder that would crash dovecot while doing a fts. While splitting up the mailbox to narrow it down, I found there are a good number of messages in that folder that all made it crash in the same manner. I figured I'd narrow down a single one, get that fixed, then retest the rest. I also added to my todo list to try a full text search on as much email as I can find, to try to weed out any other problems (and see if fts is realistic to use in production).
Nov 17 09:03:26 boomhauer dovecot: IMAP(mcdouga9): file message-decoder.c: line 201 (translation_buf_decode): assertion failed: (*size >= skip) Nov 17 09:03:26 boomhauer dovecot: child 14668 (imap) killed with signal 6
Let me know if this is not reproducable with the attached tgz containing a maildir with one message, I could do a local gdb if needed.