more robust;

--- a/Admin/Linux/Isabelle.c	Tue Jan 26 19:55:11 2021 +0100
+++ b/Admin/Linux/Isabelle.c	Tue Jan 26 20:06:38 2021 +0100
@@ -24,18 +24,19 @@
   int i = 0;
 
   dcmd = strdup(argv[0]);
+  if (dcmd == NULL) fail("Failed to allocate memory");
+
   dname = dirname(dcmd);
 
   cmd_line = malloc(sizeof(char *) * (argc + 1));
   if (cmd_line == NULL) fail("Failed to allocate memory");
 
-  cmd = cmd_line[0];
   cmd = malloc(strlen(dname) + strlen("/lib/scripts/Isabelle_app") + 1);
   if (cmd == NULL) fail("Failed to allocate memory");
   sprintf(cmd, "%s/lib/scripts/Isabelle_app", dname);
 
+  cmd_line[0] = cmd;
   for (i = 1; i < argc; i++) cmd_line[i] = argv[i];
-
   cmd_line[argc] = NULL;
 
   execvp(cmd, cmd_line);