AU: Beginnings of delta support

src/platform/update_engine/decompressing_file_writer.cc

Index: src/platform/update_engine/decompressing_file_writer.cc
diff --git a/src/platform/update_engine/decompressing_file_writer.cc b/src/platform/update_engine/decompressing_file_writer.cc
index 15bd4fd66372802b3a03bf744f847814131597a7..96db0100b18dbab125b4cd8a66f8f14ed20e8fa1 100644
--- a/src/platform/update_engine/decompressing_file_writer.cc
+++ b/src/platform/update_engine/decompressing_file_writer.cc
@@ -37,7 +37,10 @@ int GzipDecompressingFileWriter::Write(const void* bytes, size_t count) {
   // so not all data we get fed to us this time will necessarily
   // be written out this time (in decompressed form).
 
-  CHECK_EQ(0, stream_.avail_in);
+  if (stream_.avail_in) {
+    LOG(ERROR) << "Have data already. Bailing";
+    return -1;
+  }
   char buf[1024];
 
   buffer_.reserve(count);