modules/zstd/frame_header_test: Fix absl::Span handling

Internal-tag: [#64294]
Signed-off-by: Pawel Czarnecki <[email protected]>

xls/modules/zstd/frame_header_test.cc

@@ -58,9 +58,19 @@ enum FrameHeaderStatus : uint8_t {
 };
 
 struct ZstdFrameHeader {
-  absl::Span<const uint8_t> kBuffer;
   ZSTD_frameHeader kHeader;
   size_t kResult;
+
+  absl::Span<const uint8_t> kBuffer() const {
+    return absl::MakeConstSpan(buffer_);
+  }
+
+  ZstdFrameHeader(absl::Span<const uint8_t> buffer, ZSTD_frameHeader& h,
+                  size_t& r)
+      : kHeader(h), kResult(r) {
+    std::vector<uint8_t> v(buffer.begin(), buffer.end());
+    buffer_ = v;
+  }
   // Parse a frame header from an arbitrary buffer with the ZSTD library.
   static absl::StatusOr<ZstdFrameHeader> Parse(
       absl::Span<const uint8_t> buffer) {
@@ -71,6 +81,9 @@ struct ZstdFrameHeader {
         &zstd_fh, buffer.data(), buffer.size(), ZSTD_f_zstd1_magicless);
     return ZstdFrameHeader(buffer, zstd_fh, result);
   }
+
+ private:
+  std::vector<uint8_t> buffer_;
 };
 
 class FrameHeaderTest : public xls::IrTestBase {
@@ -113,7 +126,7 @@ class FrameHeaderTest : public xls::IrTestBase {
   // expected values.
   void RunAndExpectFrameHeader(const ZstdFrameHeader& zstd_frame_header) {
     // Extend buffer contents to 128 bits if necessary.
-    const absl::Span<const uint8_t>& buffer = zstd_frame_header.kBuffer;
+    const absl::Span<const uint8_t>& buffer = zstd_frame_header.kBuffer();
     std::vector<uint8_t> buffer_extended(kDslxBufferSizeBytes, 0);
     absl::Span<const uint8_t> input_buffer;
     if (buffer.size() < kDslxBufferSizeBytes) {