fuzz: Add missing ECC_Start to key_io test

Summary: Backport of core PR18319.

Test Plan:

ninja bitcoin-fuzzers
./src/test/fuzz/key_io <path_to_corpus>

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8962