Check if DnsCache is null in DnsNameResolver constructor, close AsyncHttpClient#1328

Motivation:

We miss checking if DnsCache is null in DnsNameResolver constructor
which will later then lead to a NPE. Better fail fast here.

Modifications:

Check for null and if so throw a NPE.

Result:

Fail fast.

Author: slandelle

netty-bp/resolver-dns/src/main/java/io/netty/resolver/dns/DnsNameResolver.java

@@ -196,7 +196,7 @@ public DnsNameResolver(
         this.maxPayloadSize = checkPositive(maxPayloadSize, "maxPayloadSize");
         this.optResourceEnabled = optResourceEnabled;
         this.hostsFileEntriesResolver = checkNotNull(hostsFileEntriesResolver, "hostsFileEntriesResolver");
-        this.resolveCache = resolveCache;
+        this.resolveCache = checkNotNull(resolveCache, "resolveCache");
         this.searchDomains = checkNotNull(searchDomains, "searchDomains").clone();
         this.ndots = checkPositiveOrZero(ndots, "ndots");