Fix r and s check in is_valid_signature

[arhag]

The && should be an || in:

silkworm/silkworm/core/crypto/secp256k1n.cpp

Line 25 in 05cde07

if (r >= kSecp256k1n && s >= kSecp256k1n) {

The conditions in which the is_valid_signature would fail when corrected still fail later on in the code. This bug fix just makes the failure happen earlier.