Take the output adapter by reference at the serializer ctor

Per review: the serializer still holds the adapter as a non-owning
pointer, but the constructor now takes output_adapter_protocol<char>&
and takes its address internally, so every call site passes a
reference. A reference cannot be null and reads as a borrow, which
makes the lifetime contract harder to get wrong than handing over a
raw pointer. The stored member and the write path are unchanged.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XAYM1qhSA2FDaDcGfPW3fG
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/json.hpp

@@ -1347,13 +1347,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
         if (indent >= 0)
         {
-            serializer s(&string_adapter, indent_char,
+            serializer s(string_adapter, indent_char,
                          true, ensure_ascii, static_cast<std::size_t>(indent), error_handler);
             s.dump(*this);
         }
         else
         {
-            serializer s(&string_adapter, indent_char,
+            serializer s(string_adapter, indent_char,
                          false, ensure_ascii, 0, error_handler);
             s.dump(*this);
         }
@@ -4085,7 +4085,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
         // do the actual serialization
         detail::output_stream_adapter<char> stream_adapter(o);
-        serializer s(&stream_adapter, o.fill(),
+        serializer s(stream_adapter, o.fill(),
                      pretty_print, false, static_cast<std::size_t>(indentation));
         s.dump(j);
         return o;