fix accidental use after free

Author: ironage

src/realm/sync/network/network_ssl.cpp

@@ -206,13 +206,16 @@ std::string SecureTransportErrorCategory::message(int value) const
 {
     const char* message = "Unknown error";
 #if REALM_HAVE_SECURE_TRANSPORT
+    std::unique_ptr<char[]> buffer;
     if (__builtin_available(iOS 11.3, macOS 10.3, tvOS 11.3, watchOS 4.3, *)) {
         auto status = OSStatus(value);
         void* reserved = nullptr;
-        std::unique_ptr<char[]> buffer;
         if (auto cf_message = adoptCF(SecCopyErrorMessageString(status, reserved)))
             message = cfstring_to_cstring(cf_message.get(), buffer);
     }
+    else {
+        static_cast<void>(buffer);
+    }
 #endif // REALM_HAVE_SECURE_TRANSPORT
 
     return util::format("SecureTransport error: %1 (%2)", message, value); // Throws