Add SSL_CTX_get_keylog_callback.

Conscrypt would like to write a CTS test that the callback isn't set
unexpectedly.

Change-Id: I11f987422daf0544e90f5cff4d7aaf557ac1f5a2
Reviewed-on: https://boringssl-review.googlesource.com/11060
Reviewed-by: Kenny Root <kroot@google.com>
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 04a1411..0978d75 100644
--- a/ssl/ssl_lib.c
+++ b/ssl/ssl_lib.c
@@ -2524,6 +2524,11 @@
   ctx->keylog_callback = cb;
 }
 
+void (*SSL_CTX_get_keylog_callback(const SSL_CTX *ctx))(const SSL *ssl,
+                                                        const char *line) {
+  return ctx->keylog_callback;
+}
+
 void SSL_CTX_set_current_time_cb(SSL_CTX *ctx,
                                  void (*cb)(const SSL *ssl,
                                             struct timeval *out_clock)) {