Fix bssl client -session-out.

It was switched to using the callbacks, but the callbacks require
SSL_SESS_CACHE_CLIENT be set.

(We should possibly just stop conditioning the callback on that bit
since it doesn't do anything.)

Change-Id: I96277b8a75975a86393260a6291eafc9cb2c6a78
Reviewed-on: https://boringssl-review.googlesource.com/10805
Reviewed-by: Steven Valdez <svaldez@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>
diff --git a/tool/client.cc b/tool/client.cc
index 9893cd4..27084fc 100644
--- a/tool/client.cc
+++ b/tool/client.cc
@@ -256,10 +256,12 @@
   if (args_map.count("-session-out") != 0) {
     session_out.reset(BIO_new_file(args_map["-session-out"].c_str(), "wb"));
     if (!session_out) {
-      fprintf(stderr, "Error while saving session:\n");
+      fprintf(stderr, "Error while opening %s:\n",
+              args_map["-session-out"].c_str());
       ERR_print_errors_cb(PrintErrorCallback, stderr);
       return false;
     }
+    SSL_CTX_set_session_cache_mode(ctx.get(), SSL_SESS_CACHE_CLIENT);
     SSL_CTX_sess_set_new_cb(ctx.get(), NewSessionCallback);
   }