OCSP stapling: properly check if there is ssl.ctx.

This fixes segfault if stapling was enabled in a server without a certificate
configured (and hence no ssl.ctx).

src/http/modules/ngx_http_ssl_module.c

diff --git a/src/http/modules/ngx_http_ssl_module.c b/src/http/modules/ngx_http_ssl_module.c
index 284edd0..6288283 100644
--- a/src/http/modules/ngx_http_ssl_module.c
+++ b/src/http/modules/ngx_http_ssl_module.c
@@ -737,7 +737,7 @@
 
         sscf = cscfp[s]->ctx->srv_conf[ngx_http_ssl_module.ctx_index];
 
-        if (!sscf->stapling) {
+        if (sscf->ssl.ctx == NULL || !sscf->stapling) {
             continue;
         }