AU: Check the delta magic and fail on mismatch.

This patch also fixes an issue where update_engine may keep the rootfs/kernel
file handles open when it fails to apply an update and the delta performer is
closed prematurely.

BUG=7645
TEST=unit tests, tested on device

Change-Id: If5706e0f5dd69fb728d97fc35c83f25cba144c4d

Review URL: http://codereview.chromium.org/5121008
diff --git a/delta_performer_unittest.cc b/delta_performer_unittest.cc
index 37a0c68..42f237b 100755
--- a/delta_performer_unittest.cc
+++ b/delta_performer_unittest.cc
@@ -367,6 +367,17 @@
   DoSmallImageTest(false, false, true);
 }
 
+TEST(DeltaPerformerTest, BadDeltaMagicTest) {
+  PrefsMock prefs;
+  DeltaPerformer performer(&prefs);
+  EXPECT_EQ(0, performer.Open("/dev/null", 0, 0));
+  EXPECT_TRUE(performer.OpenKernel("/dev/null"));
+  EXPECT_EQ(4, performer.Write("junk", 4));
+  EXPECT_EQ(8, performer.Write("morejunk", 8));
+  EXPECT_LT(performer.Write("morejunk", 8), 0);
+  EXPECT_LT(performer.Close(), 0);
+}
+
 TEST(DeltaPerformerTest, IsIdempotentOperationTest) {
   DeltaArchiveManifest_InstallOperation op;
   EXPECT_TRUE(DeltaPerformer::IsIdempotentOperation(op));