Add |SSL_CTX_set0_buffer_pool|.

This currently only works for certificates parsed from the network, but
if making several connections that share certificates, some KB of memory
might be saved.

BUG=chromium:671420

Change-Id: I1c7a71d84e1976138641f71830aafff87f795f9d
Reviewed-on: https://boringssl-review.googlesource.com/12706
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/ssl_lib.c

diff --git a/ssl/ssl_lib.c b/ssl/ssl_lib.c
index 8638396..e5c0559 100644
--- a/ssl/ssl_lib.c
+++ b/ssl/ssl_lib.c
@@ -1070,6 +1070,10 @@
 
 uint32_t SSL_get_mode(const SSL *ssl) { return ssl->mode; }
 
+void SSL_CTX_set0_buffer_pool(SSL_CTX *ctx, CRYPTO_BUFFER_POOL *pool) {
+  ctx->pool = pool;
+}
+
 X509 *SSL_get_peer_certificate(const SSL *ssl) {
   if (ssl == NULL) {
     return NULL;