Initialized trivia fields of QpackInstructionWithValues

Default constructor does not do that.

Report: http://sponge2/5a637df8-d10e-48b6-938c-b07fc32889be

This is cleanup for
-Xclang -enable-noundef-analysis
-mllvm -msan-eager-checks=1

PiperOrigin-RevId: 406223249
diff --git a/quic/core/qpack/qpack_instructions.h b/quic/core/qpack/qpack_instructions.h
index fcf8f22..56613de 100644
--- a/quic/core/qpack/qpack_instructions.h
+++ b/quic/core/qpack/qpack_instructions.h
@@ -194,10 +194,10 @@
   QpackInstructionWithValues() = default;
 
   // |*instruction| is not owned.
-  const QpackInstruction* instruction_;
-  bool s_bit_;
-  uint64_t varint_;
-  uint64_t varint2_;
+  const QpackInstruction* instruction_ = nullptr;
+  bool s_bit_ = false;
+  uint64_t varint_ = 0;
+  uint64_t varint2_ = 0;
   absl::string_view name_;
   absl::string_view value_;
 };