pingpong: Check SSL library buffers for already read data

Otherwise the connection can get stuck during various phases, waiting
for new data on the socket using select() etc., but it will never be
received as the data has already been read into SSL library.

lib/pingpong.c

@@ -33,6 +33,7 @@
 #include "pingpong.h"
 #include "multiif.h"
 #include "non-ascii.h"
+#include "sslgen.h"
 
 #define _MPRINTF_REPLACE /* use our functions only */
 #include <curl/mprintf.h>
@@ -105,6 +106,9 @@ CURLcode Curl_pp_statemach(struct pingpong *pp, bool block)
   if(Curl_pp_moredata(pp))
     /* We are receiving and there is data in the cache so just read it */
     rc = 1;
+  else if(!pp->sendleft && Curl_ssl_data_pending(conn, FIRSTSOCKET))
+    /* We are receiving and there is data ready in the SSL library */
+    rc = 1;
   else
     rc = Curl_socket_ready(pp->sendleft?CURL_SOCKET_BAD:sock, /* reading */
                            pp->sendleft?sock:CURL_SOCKET_BAD, /* writing */