Null-check SSL_HANDSHAKE first in SSL_serialize_handshake_hints

After boringssl-review.googlesource.com/c/boringssl/+/97127, if
SSL_serialize_handshake_hints is erroneously called for a client, the
SSL_HANDSHAKE gets dereferenced which may cause a crash due to a NULL
deref. This is an error anyway, but the aforementioned CL did not intend
to change any behavior, so return an error first rather than potentially
crashing if called for a client, and also NULL-check the SSL_HANDSHAKE.

Change-Id: I4cc241757667ae2cc18e45827182e14d6a6a6964
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/98187
Reviewed-by: David Benjamin <davidben@google.com>
Presubmit-BoringSSL-Verified: boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com <boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Lily Chen <chlily@google.com>

ssl/handoff.cc

diff --git a/ssl/handoff.cc b/ssl/handoff.cc
index 7f346c3..67ce1e5 100644
--- a/ssl/handoff.cc
+++ b/ssl/handoff.cc
@@ -937,12 +937,12 @@
 
 int SSL_serialize_handshake_hints(const SSL *ssl, CBB *out) {
   const SSL_HANDSHAKE *hs = ssl->s3->hs.get();
-  const SSL_HANDSHAKE_HINTS *const hints = hs->pending_hints.get();
-  if (!ssl->server || hints == nullptr) {
+  if (!ssl->server || hs == nullptr || hs->pending_hints == nullptr) {
     OPENSSL_PUT_ERROR(SSL, ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED);
     return 0;
   }
 
+  const SSL_HANDSHAKE_HINTS *const hints = hs->pending_hints.get();
   CBB seq, child;
   if (!CBB_add_asn1(out, &seq, CBS_ASN1_SEQUENCE)) {
     return 0;