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.cc b/payload_consumer/xz_extent_writer.cc
index a648351..361e635 100644
--- a/payload_consumer/xz_extent_writer.cc
+++ b/payload_consumer/xz_extent_writer.cc
@@ -53,13 +53,13 @@
 }  // namespace
 
 XzExtentWriter::~XzExtentWriter() {
-  xz_dec_end(stream_);
+  stream_.reset();
   TEST_AND_RETURN(input_buffer_.empty());
 }
 
 bool XzExtentWriter::Init(const RepeatedPtrField<Extent>& extents,
                           uint32_t block_size) {
-  stream_ = xz_dec_init(XZ_DYNALLOC, kXzMaxDictSize);
+  stream_.reset(xz_dec_init(XZ_DYNALLOC, kXzMaxDictSize));
   TEST_AND_RETURN_FALSE(stream_ != nullptr);
   return underlying_writer_->Init(extents, block_size);
 }
@@ -86,7 +86,7 @@
   for (;;) {
     request.out_pos = 0;
 
-    xz_ret ret = xz_dec_run(stream_, &request);
+    xz_ret ret = xz_dec_run(stream_.get(), &request);
     if (ret != XZ_OK && ret != XZ_STREAM_END) {
       LOG(ERROR) << "xz_dec_run returned " << XzErrorString(ret);
       return false;