tac: avoid double free

* src/tac.c (main): Reading a line longer than 16KiB would cause
tac to realloc its primary buffer.  Then, just before exit, tac
would mistakenly free the original (now free'd) buffer.
This bug was introduced by commit be6c13e7, "maint: always free a
buffer, to avoid even semblance of a leak".
* NEWS (Bug fixes): Mention it.
* tests/misc/tac (double-free): New test, to exercise this.
Reported by Salvo Tomaselli in <http://bugs.debian.org/594666>.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/tac.c b/src/tac.c
index cec973601..859e0067a 100644
--- a/src/tac.c
+++ b/src/tac.c
@@ -633,7 +633,6 @@ main (int argc, char **argv)
   if (! (read_size < half_buffer_size && half_buffer_size < G_buffer_size))
     xalloc_die ();
   G_buffer = xmalloc (G_buffer_size);
-  void *buf = G_buffer;
   if (sentinel_length)
     {
       strcpy (G_buffer, separator);
@@ -666,6 +665,9 @@ main (int argc, char **argv)
       error (0, errno, "-");
       ok = false;
     }
-  free (buf);
+
+  size_t offset = sentinel_length ? sentinel_length : 1;
+  free (G_buffer - offset);
+
   exit (ok ? EXIT_SUCCESS : EXIT_FAILURE);
 }