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
YY_BUFFER_STATE re_yy_scan_string (yyconst char * yystr , yyscan_t yyscanner)
{
    
	return re_yy_scan_bytes(yystr,strlen(yystr) ,yyscanner);
}

YY_BUFFER_STATE re_yy_scan_string (yyconst char * yystr , yyscan_t yyscanner)
{
    
	return re_yy_scan_bytes(yystr,strlen(yystr) ,yyscanner);
}
