Set the global pointer to the precomputation table only after initializing
it completely, and unset it before doing any uninitialization.
This causes fail-fast behavior in case of race conditions between
initialization and operations using it.
This makes a basic effort and has not been audited.
Doesn't appear to have a measurable performance impact on bench.
It also adds a secp256k1_num_free to secp256k1_ecdsa_pubkey_create.