Fix a crash when calling `EstimatedMemoryUsage()` on an empty checksummed `absl::Cord`.

PiperOrigin-RevId: 567695227
Change-Id: I13eb8a1872d2fe703b5f3b9bc8df7fec4381fb55

absl/strings/cord_analysis.cc

@@ -157,6 +157,9 @@ size_t GetEstimatedUsage(const CordRep* rep) {
   // Consume the top level CRC node if present.
   if (repref.rep->tag == CRC) {
     raw_usage.Add(sizeof(CordRepCrc), repref);
+    if (repref.rep->crc()->child == nullptr) {
+      return static_cast<size_t>(raw_usage.total);
+    }
     repref = repref.Child(repref.rep->crc()->child);
   }
 

absl/strings/cord_test.cc

@@ -3240,6 +3240,12 @@ TEST_P(CordTest, ChecksummedEmptyCord) {
   EXPECT_EQ(absl::HashOf(c3), absl::HashOf(absl::string_view()));
 }
 
+TEST(CrcCordTest, ChecksummedEmptyCordEstimateMemoryUsage) {
+  absl::Cord cord;
+  cord.SetExpectedChecksum(0);
+  EXPECT_NE(cord.EstimatedMemoryUsage(), 0);
+}
+
 #if defined(GTEST_HAS_DEATH_TEST) && defined(ABSL_INTERNAL_CORD_HAVE_SANITIZER)
 
 // Returns an expected poison / uninitialized death message expression.