query: fix non full clustering key deserialization

Clustering key prefix may have less columns than described in schema.
Deserailiaztion should stop when end of buffer is reached.

Message-Id: <20160503140420.GP23113@scylladb.com>

query-result-set.cc

@@ -141,6 +141,9 @@ result_set_builder::deserialize(const clustering_key& key)
     std::unordered_map<sstring, data_value> cells;
     auto i = key.begin(*_schema);
     for (auto&& col : _schema->clustering_key_columns()) {
+        if (i == key.end(*_schema)) {
+            break;
+        }
         cells.emplace(col.name_as_text(), col.type->deserialize_value(*i));
         ++i;
     }