#265: fix critical errors

enzevalos_iphone/OutgoingMail.swift

@@ -116,11 +116,13 @@ class OutgoingMail {
         self.sendEncryptedIfPossible = sendEncryptedIfPossible
         self.attachments = attachments
         self.orderReceivers()
-        let signature = OutgoingMail.extractPGPSignature(attachment: attachments.first!)
-        guard let SignatureData =  signature.first?.data(using: .utf8) else {
-            return
+        if let attachment = attachments.first {
+            let signature = OutgoingMail.extractPGPSignature(attachment: attachment)
+            if let signatureData =  signature.first?.data(using: .utf8) {
+                self.attachedSignature = signatureData
+            }
         }
-        self.attachedSignature = SignatureData
+        
     }
     
     init(mail: PersistentMail){