Fix bug where uncompressing key removes prefix

This method is supposed to remove the prefix if the argument
is false. However, it does so indiscriminately. As a result, it can
sometimes give the public key have 63 bytes. This commit fixes
it by checking to see if removing the prefix would result in the
public key having 63 bytes.

Author: MasterJ93

Sources/ATCryptography/p256/P256Encoding.swift

@@ -86,7 +86,7 @@ public struct P256Encoding {
             }
         } else {
             // Ensure the key is returned without the prefix.
-            if uncompressedKey.first == 0x04 {
+            if uncompressedKey.first == 0x04 && (uncompressedKey.count - 1) != 63 {
                 uncompressedKey.removeFirst()
             }
         }