Switch some pointer arithmetic to spans

Change-Id: I881c69040a71cbe82a90e8a0074af1fdad6d046a
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/67827
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Bob Beck <bbe@google.com>
Auto-Submit: David Benjamin <davidben@google.com>
diff --git a/ssl/handshake_server.cc b/ssl/handshake_server.cc
index 06d9025..1a25ea7 100644
--- a/ssl/handshake_server.cc
+++ b/ssl/handshake_server.cc
@@ -228,8 +228,7 @@
       } else if (client_hello->version <= DTLS1_VERSION) {
         versions_len = 2;
       }
-      CBS_init(&versions, kDTLSVersions + sizeof(kDTLSVersions) - versions_len,
-               versions_len);
+      versions = MakeConstSpan(kDTLSVersions).last(versions_len);
     } else {
       if (client_hello->version >= TLS1_2_VERSION) {
         versions_len = 6;
@@ -238,8 +237,7 @@
       } else if (client_hello->version >= TLS1_VERSION) {
         versions_len = 2;
       }
-      CBS_init(&versions, kTLSVersions + sizeof(kTLSVersions) - versions_len,
-               versions_len);
+      versions = MakeConstSpan(kTLSVersions).last(versions_len);
     }
   }