tac: avoid out of bounds access

This was flagged on CheriBSD on ARM Morello with the error:
"In-address space security exception (core dumped)"
triggered with: tac -s '' /dev/null

* src/tac.c (main): Ensure we don't read beyond the
end of the supplied optarg.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/tac.c b/src/tac.c
index e4aac77fe..f086f5345 100644
--- a/src/tac.c
+++ b/src/tac.c
@@ -553,7 +553,7 @@ main (int argc, char **argv)
   G_buffer = xmalloc (G_buffer_size);
   if (sentinel_length)
     {
-      memcpy (G_buffer, separator, sentinel_length + 1);
+      memcpy (G_buffer, separator, sentinel_length + !!*separator);
       G_buffer += sentinel_length;
     }
   else