Fix truncation check in (deprecated) mg_upload_field_found
Fix truncation check in (deprecated) mg_upload_field_found

The logic here is inverted. If 'truncated' has been set to 'true' by
mg_snprintf(), then it means the file path was too long.

GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver