Store Smtp.error as Option<String>

Without this change, when SMTP password is incorrect,
as_str(sock.error) is called with a null pointer,
and as_str panics.

Now it does not crash when the error is not set.

src/smtp.rs

@@ -12,7 +12,7 @@ pub struct Smtp {
     transport_connected: bool,
     /// Email address we are sending from.
     from: Option<EmailAddress>,
-    pub error: *mut libc::c_char,
+    pub error: Option<String>,
 }
 
 impl Smtp {
@@ -22,7 +22,7 @@ impl Smtp {
             transport: None,
             transport_connected: false,
             from: None,
-            error: std::ptr::null_mut(),
+            error: None,
         }
     }