SSL: take into account data in the buffer while limiting output.

In some rare cases this can result in a more smooth sending rate.

src/event/ngx_event_openssl.c

diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
index d288bc8..1a1498c 100644
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -1168,7 +1168,7 @@
         buf->end = buf->start + NGX_SSL_BUFSIZE;
     }
 
-    send = 0;
+    send = buf->last - buf->pos;
     flush = (in == NULL) ? 1 : buf->flush;
 
     for ( ;; ) {