if in doubt, prefer unwrap_or_default()

if the past we had lots of crashes because of unexpected unwrap failures,
mostly related to string.
this commit avoids them eg. for string-conversions that may panic
eg. when encountering a null-byte or by logical programming errors
where an object is assumed to be set but is not under unexpected circumstances.

src/mimefactory.rs

@@ -695,7 +695,7 @@ impl<'a> MimeFactory<'a> {
                         Ok(())
                     },
                 )
-                .unwrap();
+                .unwrap_or_default();
 
             let command = factory.msg.param.get_cmd();
             let msg = &factory.msg;
@@ -900,7 +900,7 @@ fn build_body_file(
         let mime_sub = mailmime_new_empty(content, mime_fields);
         let abs_path = dc_get_abs_path(context, path_filename)
             .to_c_string()
-            .unwrap();
+            .unwrap_or_default();
         mailmime_set_body_file(mime_sub, dc_strdup(abs_path.as_ptr()));
         Ok((mime_sub, filename_to_send))
     }