Noticed by Nils with: -g raw and this configuration:

cformat=
rformat=fixed_string

That would have the effect of just emitting the fixed string for every raw transaction (possibly filtered by a query) -- obviously very much a corner case, but nevertheless varnishevent shouldn't crash.

#2  0x0000000000404e41 in assert_fail (func=0x411031 <__func__.6111> "main", file=0x41090a "varnishevent.c", line=930, 
    cond=0x410580 "!VSTAILQ_EMPTY(&rdr_rec_freelist) || (!VSB_EMPTY(config.cformat) && nonrecs_wanted[VSL_t_req]) || (!VSB_EMPTY(config.bformat) && nonrecs_wanted[VSL_t_bereq]) || (!VSB_EMPTY(config.rformat) && nonrecs_"..., err_e=<optimized out>) at varnishevent.c:569
#3  0x0000000000404615 in main (argc=<optimized out>, argv=<optimized out>) at varnishevent.c:927