indent; add comment

This re-applies 40a87c4c08be0cdd87a3df283f285b3c2a0c8445 which got
accidentally reverted in 75134c6c86e54c10fd9e11c4345aadcdabc0f8fb.
Thanks asn for spotting this.

src/common/tortls.c

@@ -580,6 +580,8 @@ tor_tls_create_certificate(crypto_pk_env_t *rsa,
                            const char *cname_sign,
                            unsigned int cert_lifetime)
 {
+  /* OpenSSL generates self-signed certificates with random 64-bit serial
+   * numbers, so let's do that too. */
 #define SERIAL_NUMBER_SIZE 8
 
   time_t start_time, end_time;
@@ -607,12 +609,12 @@ tor_tls_create_certificate(crypto_pk_env_t *rsa,
     goto error;
 
   { /* our serial number is 8 random bytes. */
-  if (crypto_rand((char *)serial_tmp, sizeof(serial_tmp)) < 0)
-    goto error;
-  if (!(serial_number = BN_bin2bn(serial_tmp, sizeof(serial_tmp), NULL)))
-    goto error;
-  if (!(BN_to_ASN1_INTEGER(serial_number, X509_get_serialNumber(x509))))
-    goto error;
+    if (crypto_rand((char *)serial_tmp, sizeof(serial_tmp)) < 0)
+      goto error;
+    if (!(serial_number = BN_bin2bn(serial_tmp, sizeof(serial_tmp), NULL)))
+      goto error;
+    if (!(BN_to_ASN1_INTEGER(serial_number, X509_get_serialNumber(x509))))
+      goto error;
   }
 
   if (!(name = tor_x509_name_new(cname)))