secp256k1_ec_privkey_negate and invalid privkeys #488
Description
For keys greater than the order, secp256k1_scalar_negate returns strange results (understandable!).
But, as a result, secp256k1_ec_privkey_negate, when called with an invalid private key, returns 1 (OK), and a ?private key?.
Tripped me up for a bit when generating some test fixtures for secp256k1_ec_privkey_negate.
0000000000000000000000000000000000000000000000000000000000000000 becomes 0000000000000000000000000000000000000000000000000000000000000000
ff00000000000000000000000000000000000000000000000000000000000000 becomes 00fffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141
ffff000000000000000000000000000000000000000000000000000000000000 becomes 0000fffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141
ffffff0000000000000000000000000000000000000000000000000000000000 becomes 000000fffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141
ffffffff00000000000000000000000000000000000000000000000000000000 becomes 00000000fffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141
ffffffffff000000000000000000000000000000000000000000000000000000 becomes 0000000000fffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141
ffffffffffff0000000000000000000000000000000000000000000000000000 becomes 000000000000fffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141
ffffffffffffff00000000000000000000000000000000000000000000000000 becomes 00000000000000fffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141
ffffffffffffffff000000000000000000000000000000000000000000000000 becomes 0000000000000000fffffffffffffffebaaedce6af48a03bbfd25e8cd0364141
ffffffffffffffffff0000000000000000000000000000000000000000000000 becomes 000000000000000000fffffffffffffebaaedce6af48a03bbfd25e8cd0364141
ffffffffffffffffffff00000000000000000000000000000000000000000000 becomes 00000000000000000000fffffffffffebaaedce6af48a03bbfd25e8cd0364141
ffffffffffffffffffffff000000000000000000000000000000000000000000 becomes 0000000000000000000000fffffffffebaaedce6af48a03bbfd25e8cd0364141
ffffffffffffffffffffffff0000000000000000000000000000000000000000 becomes 000000000000000000000000fffffffebaaedce6af48a03bbfd25e8cd0364141
ffffffffffffffffffffffffff00000000000000000000000000000000000000 becomes 00000000000000000000000000fffffebaaedce6af48a03bbfd25e8cd0364141
ffffffffffffffffffffffffffff000000000000000000000000000000000000 becomes 0000000000000000000000000000fffebaaedce6af48a03bbfd25e8cd0364141
ffffffffffffffffffffffffffffff0000000000000000000000000000000000 becomes 000000000000000000000000000000febaaedce6af48a03bbfd25e8cd0364141
ffffffffffffffffffffffffffffffff00000000000000000000000000000000 becomes fffffffffffffffffffffffffffffffe755db9cd5e9140777fa4bd19a06c8282
ffffffffffffffffffffffffffffffffff000000000000000000000000000000 becomes fffffffffffffffffffffffffffffffd765db9cd5e9140777fa4bd19a06c8282
ffffffffffffffffffffffffffffffffffff0000000000000000000000000000 becomes fffffffffffffffffffffffffffffffd755eb9cd5e9140777fa4bd19a06c8282
ffffffffffffffffffffffffffffffffffffff00000000000000000000000000 becomes fffffffffffffffffffffffffffffffd755dbacd5e9140777fa4bd19a06c8282
ffffffffffffffffffffffffffffffffffffffff000000000000000000000000 becomes fffffffffffffffffffffffffffffffd755db9ce5e9140777fa4bd19a06c8282
ffffffffffffffffffffffffffffffffffffffffff0000000000000000000000 becomes fffffffffffffffffffffffffffffffd755db9cd5f9140777fa4bd19a06c8282
ffffffffffffffffffffffffffffffffffffffffffff00000000000000000000 becomes fffffffffffffffffffffffffffffffd755db9cd5e9240777fa4bd19a06c8282
ffffffffffffffffffffffffffffffffffffffffffffff000000000000000000 becomes fffffffffffffffffffffffffffffffd755db9cd5e9141777fa4bd19a06c8282
ffffffffffffffffffffffffffffffffffffffffffffffff0000000000000000 becomes fffffffffffffffffffffffffffffffd755db9cd5e9140787fa4bd19a06c8282
ffffffffffffffffffffffffffffffffffffffffffffffffff00000000000000 becomes fffffffffffffffffffffffffffffffd755db9cd5e91407780a4bd19a06c8282
ffffffffffffffffffffffffffffffffffffffffffffffffffff000000000000 becomes fffffffffffffffffffffffffffffffd755db9cd5e9140777fa5bd19a06c8282
ffffffffffffffffffffffffffffffffffffffffffffffffffffff0000000000 becomes fffffffffffffffffffffffffffffffd755db9cd5e9140777fa4be19a06c8282
ffffffffffffffffffffffffffffffffffffffffffffffffffffffff00000000 becomes fffffffffffffffffffffffffffffffd755db9cd5e9140777fa4bd1aa06c8282
ffffffffffffffffffffffffffffffffffffffffffffffffffffffffff000000 becomes fffffffffffffffffffffffffffffffd755db9cd5e9140777fa4bd19a16c8282
ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff0000 becomes fffffffffffffffffffffffffffffffd755db9cd5e9140777fa4bd19a06d8282
ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff00 becomes fffffffffffffffffffffffffffffffd755db9cd5e9140777fa4bd19a06c8382