filed: avoid reading from ephemeral buffer

In AddFileset() the item pointer can be passed to all kinds of functions
like plugin event handlers.
As dir->msg is only short-lived and can be overwritten when
communicating with the directory (e.g. emitting a job message), it is
safer to copy the buffer before passing it down.

core/src/filed/dir_cmd.cc

@@ -1273,7 +1273,7 @@ static bool FilesetCmd(JobControlRecord* jcr)
   while (dir->recv() >= 0) {
     StripTrailingJunk(dir->msg);
     Dmsg1(500, "Fileset: %s\n", dir->msg);
-    AddFileset(jcr, dir->msg);
+    AddFileset(jcr, std::string{dir->msg}.c_str());
   }
 
   if (!TermFileset(jcr)) { return false; }