merge: fix coverity defects

 * config: expand flags typedef
 * fix Slist usage
 * nntp: check pointers
 * fix pointer checks in generate_body()
 * fix leak in imap_parse_path()
 * fix leak in mutt_attach_mail_sender()
 * fix pointer checks in rfc2047_decode()
 * check the buffer in enriched_putwc()
 * fix var type in text_enriched_handler()
 * check retval of mutt_str_atoui()
 * check the buffer in enriched_puts()
 * ensure that the buffer always exists in mutt_buffer_alloc()