re_lexer: Make reading escape sequences more robust (#586)

* Add test for issue #503

* re_lexer: Make reading escape sequences more robust

This commit fixes parsing incomplete escape sequences at the end of a
regular expression and parsing things like \xxy (invalid hex digits)
which before were silently turned into (char)255.

Close #503

* Update re_lexer.c
int re_yyget_debug  (yyscan_t yyscanner)
{
    struct yyguts_t * yyg = (struct yyguts_t*)yyscanner;
    return yy_flex_debug;
}

int re_yyget_debug  (yyscan_t yyscanner)
{
    struct yyguts_t * yyg = (struct yyguts_t*)yyscanner;
    return yy_flex_debug;
}
