Hi,
I did a quick fuzzing test of the command line tools coming with nettle.
echo "]"|sexp-conv will cause an abort call and echo "{MiM}"|sexp-conv causes an assert: sexp-conv: /var/tmp/portage/dev-libs/nettle-3.2/work/nettle-3.2/tools/input.c:128: sexp_input_start_coding: Assertion `!input->coding' failed.
Not sure how relevant these tools are, but usually abort/assert calls are debugging tools for situations in a software that should never happen on normal operations.