Merge pull request #15073 from jtattermusch/init_cares_dns_server

Fix cares resolution with default authority (for v1.11.x)

src/core/ext/filters/client_channel/resolver/dns/c_ares/dns_resolver_ares.cc

@@ -135,6 +135,7 @@ AresDnsResolver::AresDnsResolver(const ResolverArgs& args)
   if (path[0] == '/') ++path;
   name_to_resolve_ = gpr_strdup(path);
   // Get DNS server from URI authority.
+  dns_server_ = nullptr;
   if (0 != strcmp(args.uri->authority, "")) {
     dns_server_ = gpr_strdup(args.uri->authority);
   }