!!!BUG: Internal error when getting csv file record in json format.

## OS and miller version info ~~~ bash root@dataops-castle-01.internal:~# uname -a Linux dataops-castle-01.internal 6.1.0-39-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.148-1 (2025-08-26) x86_64 GNU/Linux root@dataops-castle-01.internal:~# mlr --version mlr 6.6.0 ~~~ ## Bug generation ~~~ bash root@dataops-castle-01.internal:~# mlr --icsv head -n 2 then tail -n 1 /tmp/sql_flat.out_flat.csv .cnt=1,datid=,pg_query_cnt=6,pg_query_dura_max=,state= root@dataops-castle-01.internal:~# mlr --c2j head -n 2 then tail -n 1 /tmp/sql_flat.out_flat.csv Internal coding error detected at file mlrmap_flatten_unflatten.go line 160 ~~~ ## Additional stuff File `sql_flat.out_flat.csv` were generated by `mlr` from other huge *.csv file. Contents of the `/tmp/sql_flat.out_flat.csv` file: ~~~ csv .cnt,datid,pg_query_cnt,pg_query_dura_max,state 1,5,4,4,idle 1,,6,, 1,16401,33,42,idle 1,5,1,,active ~~~