Unpoison secp outputs for MSan

The EllSwift and BIP324 ECDH helpers in key.cpp return buffers filled
by libsecp256k1, which is built without sanitizer instrumentation.
Mark those outputs initialized under MemorySanitizer so downstream
handshake code no longer sees spurious uninitialized-value reports.

Assisted-by: GitHub Copilot
Assisted-by: OpenAI GPT-5-Codex

Author: Sjors

src/key.cpp

@@ -10,6 +10,10 @@
 #include <hash.h>
 #include <random.h>
 
+#ifdef MEMORY_SANITIZER
+#include <sanitizer/msan_interface.h>
+#endif
+
 #include <secp256k1.h>
 #include <secp256k1_ellswift.h>
 #include <secp256k1_extrakeys.h>
@@ -321,6 +325,9 @@ EllSwiftPubKey CKey::EllSwiftCreate(std::span<const std::byte> ent32) const
 
     // Should always succeed for valid keys (asserted above).
     assert(success);
+#ifdef MEMORY_SANITIZER
+    __msan_unpoison(encoded_pubkey.data(), encoded_pubkey.size());
+#endif
     return {encoded_pubkey};
 }
 
@@ -341,6 +348,9 @@ ECDHSecret CKey::ComputeBIP324ECDHSecret(const EllSwiftPubKey& their_ellswift, c
                                           nullptr);
     // Should always succeed for valid keys (assert above).
     assert(success);
+#ifdef MEMORY_SANITIZER
+    __msan_unpoison(output.data(), output.size());
+#endif
     return output;
 }