Curl_single_getsock: fix hold/pause sock handling

The previous condition that checked if the socket was marked as readable
when also adding a writable one, was incorrect and didn't take the pause
bits properly into account.

lib/transfer.c

@@ -1214,10 +1214,10 @@ int Curl_single_getsock(const struct connectdata *conn,
   if((data->req.keepon & KEEP_SENDBITS) == KEEP_SEND) {
 
     if((conn->sockfd != conn->writesockfd) ||
-       !(data->req.keepon & KEEP_RECV)) {
-      /* only if they are not the same socket or we didn't have a readable
+       bitmap == GETSOCK_BLANK) {
+      /* only if they are not the same socket and we have a readable
          one, we increase index */
-      if(data->req.keepon & KEEP_RECV)
+      if(bitmap != GETSOCK_BLANK)
         sockindex++; /* increase index if we need two entries */
 
       DEBUGASSERT(conn->writesockfd != CURL_SOCKET_BAD);