Michael, do you want to give applying this upstream patch a try?
https://sourceforge.net/p/zsh/code/ci/727b493e2b782fca0f3933865faa9a0a6ab1a2c4/ Admittedly this is quite the guess, but: 1) the patch fixes a memory issue in compmatch.c, gcc indeed complains about that. 2) on eberlin (mips64el porterbox) the problem does not occur, so I don't know how to debug it...
