absl::Cord: Avoid passing null to memcpy and memset

Passing null to memcpy or memset is undefined behavior.
Our UBSAN tests were missing `-fno-sanitize-recover`, which
means UBSAN logs a warning, but the program continues,
causing the test not to fail.

`-fno-sanitize-recover` will be added once all errors are fixed.

PiperOrigin-RevId: 733395230
Change-Id: I5477507999c4c67e53ab4cf40547b17b92d478b4

Author: derekmauro

absl/strings/cord.cc

@@ -977,7 +977,9 @@ ResultType GenericCompare(const Cord& lhs, const RHS& rhs,
 
   size_t compared_size = std::min(lhs_chunk.size(), rhs_chunk.size());
   assert(size_to_compare >= compared_size);
-  int memcmp_res = ::memcmp(lhs_chunk.data(), rhs_chunk.data(), compared_size);
+  int memcmp_res = compared_size > 0 ? ::memcmp(lhs_chunk.data(),
+                                                rhs_chunk.data(), compared_size)
+                                     : 0;
   if (compared_size == size_to_compare || memcmp_res != 0) {
     return ComputeCompareResult<ResultType>(memcmp_res);
   }
@@ -1070,7 +1072,7 @@ void AppendCordToString(const Cord& src, absl::Nonnull<std::string*> dst) {
 void Cord::CopyToArraySlowPath(absl::Nonnull<char*> dst) const {
   assert(contents_.is_tree());
   absl::string_view fragment;
-  if (GetFlatAux(contents_.tree(), &fragment)) {
+  if (GetFlatAux(contents_.tree(), &fragment) && !fragment.empty()) {
     memcpy(dst, fragment.data(), fragment.size());
     return;
   }