always use to_string_lossy() for converting c-strings to String

the function to_string() is removed;
c-strings may always be badly formatted and this should never lead to a panic.

src/dc_mimeparser.rs

@@ -890,7 +890,7 @@ impl<'a> MimeParser<'a> {
             unsafe {
                 let fld_message_id = (*field).fld_data.fld_message_id;
                 if !fld_message_id.is_null() {
-                    return Some(to_string((*fld_message_id).mid_value));
+                    return Some(to_string_lossy((*fld_message_id).mid_value));
                 }
             }
         }