Merge #2576: Return error when constructing pubkey from slice

6ecc41d Return error when constructing pubkey from slice (Tobin C. Harding)

Pull request description:

  This PR fixes a bug introduced by me in #2473, and uncovered by #2563 - amazing that it was found so quickly!

  Constructing a pubkey using `PublicKey::from_slice` can fail for reasons other than just incorrect length - we should not be using `expect` but rather returning the error.

  A purist might argue that we are now returning a nested error type with an unreachable variant:

    `ParsePublicKeyError::Encoding(FromSliceError::InvalidLength)`

  Is this acceptable or do we want to further improve this?

ACKs for top commit:
  sanket1729:
    ACK 6ecc41d
  apoelstra:
    ACK 6ecc41d

Tree-SHA512: ae8299b21c4787a104f98533105308e8e7678cd5a29b78c30012982d741c05ba5f2bb1edd1d61d3a5ce028235d18c1511e1f94207479bc19e88cfec7a7ca1737

bitcoin/src/crypto/key.rs

@@ -241,14 +241,14 @@ impl FromStr for PublicKey {
                     InvalidChar(e) => ParsePublicKeyError::InvalidChar(e.invalid_char()),
                     InvalidLength(_) => unreachable!("length checked already")
                 })?;
-                Ok(PublicKey::from_slice(&bytes).expect("length checked already"))
+                Ok(PublicKey::from_slice(&bytes)?)
             },
             130 => {
                 let bytes = <[u8; 65]>::from_hex(s).map_err(|e| match e {
                     InvalidChar(e) => ParsePublicKeyError::InvalidChar(e.invalid_char()),
                     InvalidLength(_) => unreachable!("length checked already")
                 })?;
-                Ok(PublicKey::from_slice(&bytes).expect("length checked already"))
+                Ok(PublicKey::from_slice(&bytes)?)
             }
             len => Err(ParsePublicKeyError::InvalidHexLength(len)),
         }