Don't update checksum if data ptr is null.

Passing a nullptr to adler32 resets it to its initial value.

Bug: 28874264

(cherry-picked from commit 2c6b49498e2f084ce217fcd3327103eadc1e0c37)

Change-Id: I9ac1b66004d4028a67aca18057c03ba9fb494041
diff --git a/runtime/oat.cc b/runtime/oat.cc
index 80231f3..aab0e81 100644
--- a/runtime/oat.cc
+++ b/runtime/oat.cc
@@ -182,8 +182,12 @@
 
 void OatHeader::UpdateChecksum(const void* data, size_t length) {
   DCHECK(IsValid());
-  const uint8_t* bytes = reinterpret_cast<const uint8_t*>(data);
-  adler32_checksum_ = adler32(adler32_checksum_, bytes, length);
+  if (data != nullptr) {
+    const uint8_t* bytes = reinterpret_cast<const uint8_t*>(data);
+    adler32_checksum_ = adler32(adler32_checksum_, bytes, length);
+  } else {
+    DCHECK_EQ(0U, length);
+  }
 }
 
 InstructionSet OatHeader::GetInstructionSet() const {