Fix an unintialized return value.

Bug: http://b/131390872
Test: Built aosp_blueline successfully.
Change-Id: Iea5255be280dfc089a95fc25a0f988e913c18679
diff --git a/libsparse/sparse.cpp b/libsparse/sparse.cpp
index cb288c5..24c6379 100644
--- a/libsparse/sparse.cpp
+++ b/libsparse/sparse.cpp
@@ -188,7 +188,7 @@
                               int (*write)(void* priv, const void* data, size_t len,
                                            unsigned int block, unsigned int nr_blocks),
                               void* priv) {
-  int ret;
+  int ret = 0;
   int chunks;
   struct chunk_data chk;
   struct output_file* out;