keeper: fix memory leak in case of compression is used (default)

In case of compression is used, ZstdDeflatingAppendableWriteBuffer is
used, but it has a leak, since it frees ZSTD_CCtx only if there was
write while this is not correct, since it is created anyway.

This was found with jemalloc profile, keeper-bench and the following
keeper settings:

- force_sync=false
- snapshot_distance=100
- reserved_log_items=0
- rotate_log_storage_interval=100

Signed-off-by: Azat Khuzhin <a.khuzhin@semrush.com>

src/IO/ZstdDeflatingAppendableWriteBuffer.cpp

@@ -84,6 +84,8 @@ void ZstdDeflatingAppendableWriteBuffer::finalizeImpl()
 {
     if (first_write)
     {
+        /// To free cctx
+        finalizeAfter();
         /// Nothing was written
         return;
     }