Merge pull request #22 from IBM-Swift/issue.warnings

Fixed compilation warnings

Author: ianpartridge

Sources/CryptorRSA/CryptorRSADigest.swift

@@ -230,7 +230,7 @@ public extension Data {
 
 		#else
 
-			public var engine: (_ data: UnsafeRawPointer, _ len: CC_LONG, _ md: UnsafeMutablePointer<UInt8>) -> UnsafeMutablePointer<UInt8>! {
+			public var engine: (_ data: UnsafeRawPointer, _ len: CC_LONG, _ md: UnsafeMutablePointer<UInt8>) -> UnsafeMutablePointer<UInt8>? {
 
 				switch self {