GCC 15 warning for GCM implementation

[NIIBE Yutaka]

Hello,

I encountered a warning when I use GCC 15.

==========================
libgcrypt/cipher/cipher-gcm.c: In function 'do_ghash_buf.constprop':
libgcrypt/cipher/cipher-gcm.c:764:23: warning: array subscript 17 is above array bounds of 'unsigned char[16]' [-Warray-bounds=]
  764 |               memset (&c->u_mode.gcm.macbuf[unused], 0, n);
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
==========================

It's not the case actually.  The size_t type of n (unsigned) might be
the cause of confusion, I suppose.

To silence the warning, I tried this patch:
==========================
diff --git a/cipher/cipher-gcm.c b/cipher/cipher-gcm.c
index 5bb98015..ef657742 100644
--- a/cipher/cipher-gcm.c
+++ b/cipher/cipher-gcm.c
@@ -753,14 +753,12 @@ do_ghash_buf(gcry_cipher_hd_t c, byte *hash, const byte *buf,
         }
       if (!buflen)
         {
-          if (!do_padding && unused < blocksize)
+	  if (unused < blocksize)
 	    {
-	      break;
-	    }
+              if (!do_padding)
+                break;
 
-	  n = blocksize - unused;
-	  if (n > 0)
-	    {
+              n = blocksize - unused;
 	      memset (&c->u_mode.gcm.macbuf[unused], 0, n);
 	      unused = blocksize;
 	    }
==========================

That is, new code will be following:
==========================
      if (!buflen)
        {
	  if (unused < blocksize)
	    {
              if (!do_padding)
                break;

              n = blocksize - unused;
	      memset (&c->u_mode.gcm.macbuf[unused], 0, n);
	      unused = blocksize;
	    }
        }
==========================

I think that new code has clean semantics and read easier (computing
N only when the condition holds).

Any suggestions are welcome.
--