cryptsetup: check the existence of salt by salt_size > 0

Follow-up for 504d0ac.

The function may be called with non-NULL salt and salt_size == 0.

(cherry picked from commit 8c2264a)

Related: RHEL-40119

Author: yuwata

src/cryptsetup/cryptsetup-tokens/luks2-tpm2.c

@@ -58,7 +58,7 @@ int acquire_luks2_key(
                 return -ENOANO;
 
         /* If we're using a PIN, and the luks header has a salt, it better have a pin too */
-        if ((flags & TPM2_FLAGS_USE_PIN) && salt && !pin)
+        if ((flags & TPM2_FLAGS_USE_PIN) && salt_size > 0 && !pin)
                 return -ENOANO;
 
         if (pin && salt_size > 0) {