output: Avoid tickling UBSAN with memcpy(dest, NULL, 0)branch-1.4

Even though all libc handle it sanely (because size 0 says there is
nothing to copy), NULL is not a valid source pointer per a strict
reading of C, so UBSAN flags it:

+output.c:511:9: runtime error: null pointer passed as argument 2, which is declared to never be null

* src/output.c (make_room_for): Skip no-op memcpy.
Fixes: https://savannah.gnu.org/support/index.php?110809
Reported-by: Sam James

1 files changed, 2 insertions, 1 deletions

diff --git a/src/output.c b/src/output.c
index 956089b1..6dc8280a 100644
--- a/src/output.c
+++ b/src/output.c
@@ -508,7 +508,8 @@ make_room_for (int length)
       {
         char *buffer = output_diversion->u.buffer;
         output_diversion->u.buffer = xcharalloc ((size_t) wanted_size);
-        memcpy (output_diversion->u.buffer, buffer, output_diversion->used);
+        if (output_diversion->used)
+          memcpy (output_diversion->u.buffer, buffer, output_diversion->used);
         free (buffer);
       }