Ensure backward compatibility of the "checksum inception" bug

`Wallet` stores the descriptors' checksum in the database for safety.
Previously, the checksum used was a checksum of a descriptor that
already had a checksum.

This PR allows for backward-compatibility of databases created with this
bug.

src/descriptor/checksum.rs

@@ -83,7 +83,7 @@ pub fn get_checksum_bytes(mut desc: &str, exclude_hash: bool) -> Result<[u8; 8],
 
     // if input data already had a checksum, check calculated checksum against original checksum
     if let Some(original_checksum) = original_checksum {
-        if original_checksum.as_bytes() != &checksum {
+        if original_checksum.as_bytes() != checksum {
             return Err(DescriptorError::InvalidDescriptorChecksum);
         }
     }