segmentation fault with 'openssl s_client -prexit -keymatexport'

Reviewed-by: NKurt Roeckx <kurt@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1243)

apps/s_client.c

@@ -2633,7 +2633,8 @@ static void print_stuff(BIO *bio, SSL *s, int full)
 #endif
 
     SSL_SESSION_print(bio, SSL_get_session(s));
-    if (keymatexportlabel != NULL) {
+    if ((SSL_get_session(s) != NULL) &&
+        (keymatexportlabel != NULL)) {
         BIO_printf(bio, "Keying material exporter:\n");
         BIO_printf(bio, "    Label: '%s'\n", keymatexportlabel);
         BIO_printf(bio, "    Length: %i bytes\n", keymatexportlen);