absl::StrFormat: Avoid passing null to memcpy

Passing null to memcpy 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: 733466239
Change-Id: I9c2a2348638b78465b6d0880c93c11ba89a93efd

absl/strings/internal/str_format/output.cc

@@ -33,9 +33,11 @@ struct ClearErrnoGuard {
 
 void BufferRawSink::Write(string_view v) {
   size_t to_write = std::min(v.size(), size_);
-  std::memcpy(buffer_, v.data(), to_write);
-  buffer_ += to_write;
-  size_ -= to_write;
+  if (to_write > 0) {
+    std::memcpy(buffer_, v.data(), to_write);
+    buffer_ += to_write;
+    size_ -= to_write;
+  }
   total_written_ += v.size();
 }
 

absl/strings/internal/str_format/parser.h

@@ -132,8 +132,10 @@ class ParsedFormatBase {
     has_error_ = other.has_error_;
     items_ = other.items_;
     size_t text_size = items_.empty() ? 0 : items_.back().text_end;
-    data_.reset(new char[text_size]);
-    memcpy(data_.get(), other.data_.get(), text_size);
+    data_ = std::make_unique<char[]>(text_size);
+    if (text_size > 0) {
+      memcpy(data_.get(), other.data_.get(), text_size);
+    }
     return *this;
   }