Fix a bug in `Cord::{Append,Prepend}(CordBuffer)`: call

`MaybeRemoveEmptyCrcNode()`. Otherwise appending a `CordBuffer` an empty Cord
with a CRC node crashes (`RemoveCrcNode()` which increases the refcount of a
nullptr child).

Cosmetics: in `Cord::InlineRep::AppendArray()`, return early for empty `src`
before removing the empty CRC node.
PiperOrigin-RevId: 619107278
Change-Id: I4f1bc6b75c662f4678c61e3ef310e8597d62e2e1

absl/strings/cord.cc

@@ -425,8 +425,8 @@ Cord& Cord::operator=(absl::string_view src) {
 // we keep it here to make diffs easier.
 void Cord::InlineRep::AppendArray(absl::string_view src,
                                   MethodIdentifier method) {
-  MaybeRemoveEmptyCrcNode();
   if (src.empty()) return;  // memcpy(_, nullptr, 0) is undefined.
+  MaybeRemoveEmptyCrcNode();
 
   size_t appended = 0;
   CordRep* rep = tree();

absl/strings/cord.h

@@ -75,6 +75,7 @@
 #include "absl/base/internal/per_thread_tls.h"
 #include "absl/base/macros.h"
 #include "absl/base/nullability.h"
+#include "absl/base/optimization.h"
 #include "absl/base/port.h"
 #include "absl/container/inlined_vector.h"
 #include "absl/crc/internal/crc_cord_state.h"
@@ -1388,6 +1389,7 @@ inline void Cord::Prepend(absl::string_view src) {
 
 inline void Cord::Append(CordBuffer buffer) {
   if (ABSL_PREDICT_FALSE(buffer.length() == 0)) return;
+  contents_.MaybeRemoveEmptyCrcNode();
   absl::string_view short_value;
   if (CordRep* rep = buffer.ConsumeValue(short_value)) {
     contents_.AppendTree(rep, CordzUpdateTracker::kAppendCordBuffer);
@@ -1398,6 +1400,7 @@ inline void Cord::Append(CordBuffer buffer) {
 
 inline void Cord::Prepend(CordBuffer buffer) {
   if (ABSL_PREDICT_FALSE(buffer.length() == 0)) return;
+  contents_.MaybeRemoveEmptyCrcNode();
   absl::string_view short_value;
   if (CordRep* rep = buffer.ConsumeValue(short_value)) {
     contents_.PrependTree(rep, CordzUpdateTracker::kPrependCordBuffer);