webmidi: read the right EM_ASM argument when closing an input

stop_stream() passes one argument and read it as $1, so closing a Web MIDI
input threw a TypeError out of the destructor and aborted the program.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SCvayD61okH7PATstw4whi

Author: jcelerier

include/libremidi/backends/emscripten/midi_access.hpp

@@ -303,10 +303,10 @@ class midi_access_emscripten
       return;
 
     const auto& id = m_current_inputs[port_index].id;
-    EM_ASM(const id = UTF8ToString($1);
+    EM_ASM(const id = UTF8ToString($0);
 
            let input = globalThis.__libreMidi_access.inputs.get(id);
-           input.onmidimessage = undefined;, id.c_str());
+           if (input) input.onmidimessage = null;, id.c_str());
   }
 
   std::vector<observer_emscripten*> m_observers;