lib-sieve: util: edit-mail - Fix assert occurring when bad header is encountered.

In that case, the header name and middle are empty, causing:

Panic: file edit-mail.c: line 820 (edit_mail_headers_parse): assertion failed: (body_offset > 0)

Fixed by skipping the bad header for parsing altogether.

src/lib-sieve/util/edit-mail.c

@@ -767,6 +767,9 @@ static int edit_mail_headers_parse(struct edit_mail *edmail)
 			if (hdr == NULL || hdr->eoh)
 				break;
 
+			/* Skip bad headers */
+			if (hdr->name_len == 0)
+				continue;
 			/* We deny the existence of any `Content-Length:'
 			   header. This header is non-standard and it can wreak
 			   havok when the message is modified.