large_data_handler: Prepare for stopped qctx

All the large data handler methods rely on global qctx thing to
write down its notes. This creates circular dependency:

query processor -> database -> large_data_handler -> qctx -> qp

In scylla this is not a technical problem, neither qctx nor the
query processor are stopped. It is a problem in cql_test_env
that stops everything, including resetting qctx to null. To avoid
tests stepping on nullptr qctx add the explicit check.

Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>

db/large_data_handler.cc

@@ -110,6 +110,12 @@ future<> large_data_handler::maybe_delete_large_data_entries(sstables::shared_ss
 template <typename... Args>
 static future<> try_record(std::string_view large_table, const sstables::sstable& sst,  const sstables::key& partition_key, int64_t size,
         std::string_view desc, std::string_view extra_path, const std::vector<sstring> &extra_fields, Args&&... args) {
+    // FIXME  This check is for test/cql-test-env that stop qctx (it does so because
+    // it stops query processor and doesn't want us to access its freed instantes)
+    if (!db::qctx) {
+        return make_ready_future<>();
+    }
+
     sstring extra_fields_str;
     sstring extra_values;
     for (std::string_view field : extra_fields) {