Set precomputation table late and unset early.
Set precomputation table late and unset early.

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.

vlajos/secp256k1,chfast/secp256k1,vlajos/secp256k1,randy-waterhouse/secp256k1,randy-waterhouse/secp256k1,GSongHashrate/secp256k1,instagibbs/secp256k1,Gustav-Simonsson/secp256k1,libmetrocoin/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,ChrisXin/secp256k1,apoelstra/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,instagibbs/secp256k1,digitalbitbox/secp256k1,afk11/secp256k1,Bitcoin-com/BUcash,BitcoinUnlimited/BitcoinUnlimited,vlajos/secp256k1,GSongHashrate/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,marcusdiaz/BitcoinUnlimited,pmienk/secp256k1,jonasschnelli/secp256k1,ChrisXin/secp256k1,libmetrocoin/secp256k1,martindale/secp256k1-zkp,libbitcoin/secp256k1,voisine/secp256k1,voisine/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,jonasschnelli/secp256k1,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,afk11/secp256k1,martindale/secp256k1-zkp,voisine/secp256k1,01BTC10/secp256k1,Bitcoin-com/BUcash,pmienk/secp256k1,ElementsProject/secp256k1-zkp,Bitcoin-com/BUcash,BitcoinUnlimited/BitcoinUnlimited,evoskuil/secp256k1,sipa/secp256k1-zkp,apoelstra/secp256k1,bitcoin/secp256k1,marcusdiaz/BitcoinUnlimited,cddjr/BitcoinUnlimited,bitcoin/secp256k1,rustyrussell/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,01BTC10/secp256k1,martindale/secp256k1,Gustav-Simonsson/secp256k1,digitalbitbox/secp256k1,vlajos/secp256k1,digitalbitbox/secp256k1,sipa/secp256k1-zkp,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,Gustav-Simonsson/secp256k1,instagibbs/secp256k1,cddjr/BitcoinUnlimited,evoskuil/secp256k1,randy-waterhouse/secp256k1,afk11/secp256k1,BitcoinUnlimited/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,shea256/secp256k1-zkp,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,jonasschnelli/secp256k1,Justaphf/BitcoinUnlimited,GSongHashrate/secp256k1,jonasschnelli/secp256k1,BitcoinUnlimited/BitcoinUnlimited,shea256/secp256k1-zkp,sipa/secp256k1-zkp,01BTC10/secp256k1,marcusdiaz/BitcoinUnlimited,rustyrussell/secp256k1,randy-waterhouse/secp256k1,libmetrocoin/secp256k1,ElementsProject/secp256k1-zkp,bitcoin/secp256k1,Bitcoin-com/BUcash,libbitcoin/secp256k1,GSongHashrate/secp256k1,instagibbs/secp256k1,afk11/secp256k1,marcusdiaz/BitcoinUnlimited,libbitcoin/secp256k1,krzysztofwos/BitcoinUnlimited,kleetus/secp256k1,rustyrussell/secp256k1,sipa/secp256k1,krzysztofwos/BitcoinUnlimited,evoskuil/secp256k1,peterdettman/secp256k1,Bitcoin-com/BUcash,marcusdiaz/BitcoinUnlimited,sipa/secp256k1-zkp,cddjr/BitcoinUnlimited,sipa/secp256k1,martindale/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,peterdettman/secp256k1,chfast/secp256k1,instagibbs/secp256k1,libbitcoin/secp256k1,apoelstra/secp256k1,Justaphf/BitcoinUnlimited,voisine/secp256k1,chfast/secp256k1,ChrisXin/secp256k1,BitcoinUnlimited/BitcoinUnlimited,voisine/secp256k1,ChrisXin/secp256k1,Bitcoin-com/BUcash,cddjr/BitcoinUnlimited,shea256/secp256k1-zkp,martindale/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,bitcoin/secp256k1,kleetus/secp256k1,bitcoin-core/secp256k1,apoelstra/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,rustyrussell/secp256k1,chfast/secp256k1,digitalbitbox/secp256k1,krzysztofwos/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,shea256/secp256k1-zkp,bitcoin-core/secp256k1,apoelstra/secp256k1,martindale/secp256k1-zkp,BTCfork/hardfork_prototype_1_mvf-bu,pmienk/secp256k1,Justaphf/BitcoinUnlimited,cddjr/BitcoinUnlimited,martindale/secp256k1-zkp,libmetrocoin/secp256k1,marcusdiaz/BitcoinUnlimited,pmienk/secp256k1,krzysztofwos/BitcoinUnlimited,kleetus/secp256k1,bitcoin/secp256k1,sipa/secp256k1,Gustav-Simonsson/secp256k1,cddjr/BitcoinUnlimited,afk11/secp256k1,kleetus/secp256k1,sipa/secp256k1,chfast/secp256k1,pmienk/secp256k1,sipa/secp256k1-zkp,krzysztofwos/BitcoinUnlimited,libbitcoin/secp256k1,martindale/secp256k1,digitalbitbox/secp256k1,evoskuil/secp256k1,evoskuil/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,01BTC10/secp256k1,kleetus/secp256k1