debug assert in TEE filter test

Test https://github.com/mariadb-corporation/maxscale-system-test/blob/master/bug670.cpp fails with debug assert:

Core was generated by `/usr/local/mariadb-maxscale/bin/maxscale'.
Program terminated with signal 6, Aborted.
#0 0x00007fe861ccd625 in raise (sig=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
64 return INLINE_SYSCALL (tgkill, 3, pid, selftid, sig);