Make XzEtentWriter::Init() idempotent

xz_dec_init allocates memory, so avoid memory leak in calling Init()
    multiple times

Test: th
Change-Id: I2dc20f6f9127b6d7749369385f982a08c78e6e44
diff --git a/payload_consumer/xz_extent_writer.h b/payload_consumer/xz_extent_writer.h
index 70338f2..caf34ec 100644
--- a/payload_consumer/xz_extent_writer.h
+++ b/payload_consumer/xz_extent_writer.h
@@ -34,6 +34,10 @@
 namespace chromeos_update_engine {
 
 class XzExtentWriter : public ExtentWriter {
+  struct xz_deleter {
+    constexpr void operator()(xz_dec* p) { xz_dec_end(p); }
+  };
+
  public:
   explicit XzExtentWriter(std::unique_ptr<ExtentWriter> underlying_writer)
       : underlying_writer_(std::move(underlying_writer)) {}
@@ -47,7 +51,7 @@
   // The underlying ExtentWriter.
   std::unique_ptr<ExtentWriter> underlying_writer_;
   // The opaque xz decompressor struct.
-  xz_dec* stream_{nullptr};
+  std::unique_ptr<xz_dec, xz_deleter> stream_{nullptr};
   brillo::Blob input_buffer_;
 
   DISALLOW_COPY_AND_ASSIGN(XzExtentWriter);