json: work around fuzzer-induced assert crashes

json: work around fuzzer-induced assert crashes

commit 18c24d89b9977ddc0900be64fe9e95e7ac1ce896 upstream.

fuzzer can cause assert failures due to json_pack() returning a NULL
value and therefore triggering the assert(out) in __json_pack macro.

All instances I saw are due to invalid UTF-8 strings, i.e., table/chain
names with non-text characters in them.

Work around this for now, replace the assert with a plaintext error
message and return NULL instead of abort().

Signed-off-by: Florian Westphal <fw@strlen.de>