core/main: restore the correct assert about array position

'pos' is incremented after each assignment. If we use the maximum number
of arguments, we end up with pos==9 after all the assignments, and it
points to where the next value would be assigned. This position must remain
NULL.

The assert I "fixed" was intentionally introduced in
26abdc73a2 as a bugfix. So my "fix" repeated
the same error that was fixed back then.

src/core/main.c

@@ -1572,7 +1572,7 @@ static int become_shutdown(int objective, int retval) {
                 command_line[pos++] = exit_code;
         }
 
-        assert(pos < ELEMENTSOF(command_line) - 1);
+        assert(pos < ELEMENTSOF(command_line));
 
         /* The watchdog: */