Make muc_nick return a const char* const

In the documentation of the `muc_nick` function we can read:

> The nickname is owned by the chat room and should not be modified or freed

We should reflect that in the return type and `strdup` the value for the
plugin API.

Fixes: https://github.com/profanity-im/profanity/issues/2013

src/plugins/api.c

@@ -231,7 +231,8 @@ api_get_current_nick(void)
     if (current->type == WIN_MUC) {
         ProfMucWin* mucwin = (ProfMucWin*)current;
         assert(mucwin->memcheck == PROFMUCWIN_MEMCHECK);
-        return muc_nick(mucwin->roomjid);
+        const char* const nick = muc_nick(mucwin->roomjid);
+        return nick ? strdup(nick) : NULL;
     } else {
         return NULL;
     }
@@ -286,7 +287,9 @@ api_current_win_is_console(void)
 char*
 api_get_room_nick(const char* barejid)
 {
-    return muc_nick(barejid);
+    const char* const nick = muc_nick(barejid);
+
+    return nick ? strdup(nick) : NULL;
 }
 
 void

src/plugins/python_api.c

@@ -435,7 +435,9 @@ python_api_get_current_nick(PyObject* self, PyObject* args)
     char* nick = api_get_current_nick();
     disable_python_threads();
     if (nick) {
-        return Py_BuildValue("s", nick);
+        PyObject* obj = Py_BuildValue("s", nick);
+        free(nick);
+        return obj;
     } else {
         Py_RETURN_NONE;
     }