fix segfault introduced in r1780

src/event/ngx_event_openssl.c

diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
index d116209..d7fc4d2 100644
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -996,8 +996,10 @@
 void
 ngx_ssl_free_buffer(ngx_connection_t *c)
 {
-    if (ngx_pfree(c->pool, c->ssl->buf->start) == NGX_OK) {
-        c->ssl->buf->start = NULL;
+    if (c->ssl->buf && c->ssl->buf->start) {
+        if (ngx_pfree(c->pool, c->ssl->buf->start) == NGX_OK) {
+            c->ssl->buf->start = NULL;
+        }
     }
 }