tail: exit without reading input if would never output

* src/tail.c (main): If -n0 or -c0 were specified without -f,
then no data would ever be output, so exit without reading input.
* tests/tail-2/tail-n0f.sh: Augment the related test with this case.

src/tail.c

@@ -2149,6 +2149,10 @@ main (int argc, char **argv)
                      " indefinitely is ineffective"));
   }
 
+  /* Don't read anything if we'll never output anything.  */
+  if (! n_units && ! forever && ! from_start)
+    exit (EXIT_SUCCESS);
+
   F = xnmalloc (n_files, sizeof *F);
   for (i = 0; i < n_files; i++)
     F[i].name = file[i];

tests/tail-2/tail-n0f.sh

@@ -29,6 +29,11 @@ require_proc_pid_status_
 touch empty || framework_failure_
 echo anything > nonempty || framework_failure_
 
+# First verify that -[nc]0 without -f, exit without reading
+touch unreadable || framework_failure_
+chmod 0 unreadable || framework_failure_
+tail -c0 unreadable || fail=1
+tail -n0 unreadable || fail=1
 
 for inotify in ---disable-inotify ''; do
   for file in empty nonempty; do