Make sig_opts non-optional in X509 structures

This simplifies the code.
diff --git a/library/x509_crl.c b/library/x509_crl.c
index 26d351a..2191b47 100644
--- a/library/x509_crl.c
+++ b/library/x509_crl.c
@@ -626,11 +626,6 @@
     size_t n;
     char *p;
     const x509_crl_entry *entry;
-#if defined(POLARSSL_RSASSA_PSS_CERTIFICATES)
-    const void *sig_opts = crl->sig_opts;
-#else
-    const void *sig_opts = NULL;
-#endif
 
     p = buf;
     n = size;
@@ -687,7 +682,7 @@
     SAFE_SNPRINTF();
 
     ret = x509_sig_alg_gets( p, n, &crl->sig_oid1, crl->sig_pk, crl->sig_md,
-                             sig_opts );
+                             crl->sig_opts );
     SAFE_SNPRINTF();
 
     ret = snprintf( p, n, "\n" );