Peter O'Gorman found a problem with SCP downloads when the downloaded file

was 16385 bytes (16K+1) and it turned out we didn't properly always "suck
out" all data from libssh2. The effect being that libcurl would hang on the
socket waiting for data when libssh2 had in fact already read it all...

lib/transfer.c

@@ -270,7 +270,10 @@ CURLcode Curl_readrewind(struct connectdata *conn)
 
 static int data_pending(struct connectdata *conn)
 {
-  return Curl_ssl_data_pending(conn, FIRSTSOCKET);
+  /* in the case of libssh2, we can never be really sure that we have emptied
+     its internal buffers so we MUST always try until we get EAGAIN back */
+  return conn->protocol&(PROT_SCP|PROT_SFTP) ||
+    Curl_ssl_data_pending(conn, FIRSTSOCKET);
 }
 
 #ifndef MIN