reachable assertion during parse for `(*:`

[katef]

$ ./re -rpcre '(*:'
re: src/libre/parser.act:1307: struct ast *parse_re_pcre(re_getchar_fun *, void *, enum re_flags, int, struct re_err *, struct re_pos *): Assertion `!"unreached"' failed.
Aborted (core dumped)

probably we should lex that as $unsupported. i thought we did already. but whatever we do, we shouldn't be crashing about it

[silentbicycle]

Duplicate of #386.