Avoid redundant TreeMap.containsKey call in PolicyParser.read

There is a field 'Map<String, DomainEntry> domainEntries' in sun.security.provider.PolicyParser class. It's filled in 'read' method with non-null values only. In read method there is redundant 'containsKey' check:

    if (!domainEntries.containsKey(domainName)) {
        domainEntries.put(domainName, de);
    } else {
        LocalizedMessage localizedMsg = new LocalizedMessage(
            "duplicate.keystore.domain.name");
        Object[] source = {domainName};
        String msg = "duplicate keystore domain name: " +
                     domainName;
        throw new ParsingException(msg, localizedMsg, source);
    }

Instead of pair TreeMap.containsKey/TreeMap.put method calls, we can use single call TreeMap.putIfAbsent and check result for nullness.