Fix further leak of tdata (pjsip#4878)

The leak happens when
1. send_buf_pending is already in use
2. ssock_on_data_sent calls flush_circ_buf_output
3. send_buf_pending gets overwritten -> previous data in
   send_buf_pending is lost forever, its callback is never called, and
   therefore the reference to the tdata it contains is never
   decremented, preventing transport destruction

Author: azertyfun

pjlib/src/pj/ssl_sock_imp_common.c

@@ -584,7 +584,9 @@ static pj_status_t flush_circ_buf_output(pj_ssl_sock_t *ssock,
 
     /* Allocate buffer for send data */
     wdata = alloc_send_data(ssock, needed_len);
-    if (wdata == NULL) {
+    if (wdata == NULL && ssock->send_buf_pending.data_len) {
+        return PJ_ENOMEM;
+    } else if (wdata == NULL) {
         /* Oops, the send buffer is full, let's just
          * queue it for sending and return PJ_EPENDING.
          */