Fix Zac's crash (#284)

* Guard + copy

* obo

SecretAgentKit/Agent.swift

@@ -23,8 +23,8 @@ extension Agent {
 
     public func handle(reader: FileHandleReader, writer: FileHandleWriter) {
         Logger().debug("Agent handling new data")
-        let data = reader.availableData
-        guard !data.isEmpty else { return }
+        let data = Data(reader.availableData)
+        guard data.count > 4 else { return }
         let requestTypeInt = data[4]
         guard let requestType = SSHAgent.RequestType(rawValue: requestTypeInt) else {
             writer.write(OpenSSHKeyWriter().lengthAndData(of: SSHAgent.ResponseType.agentFailure.data))