transfer: don't unset writesockfd on setup of multiplexed conns

Curl_setup_transfer() can be called to setup a new individual transfer
over a multiplexed connection so it shouldn't unset writesockfd.

Bug: #2520

lib/transfer.c

@@ -2018,11 +2018,18 @@ Curl_setup_transfer(
 
   DEBUGASSERT((sockindex <= 1) && (sockindex >= -1));
 
-  /* now copy all input parameters */
-  conn->sockfd = sockindex == -1 ?
+  if(conn->bits.multiplex || conn->httpversion == 20) {
+    /* when multiplexing, the read/write sockets need to be the same! */
+    conn->sockfd = sockindex == -1 ?
+      conn->sock[writesockindex] : conn->sock[sockindex];
+    conn->writesockfd = conn->sockfd;
+  }
+  else {
+    conn->sockfd = sockindex == -1 ?
       CURL_SOCKET_BAD : conn->sock[sockindex];
-  conn->writesockfd = writesockindex == -1 ?
+    conn->writesockfd = writesockindex == -1 ?
       CURL_SOCKET_BAD:conn->sock[writesockindex];
+  }
   k->getheader = getheader;
 
   k->size = size;
@@ -2041,10 +2048,10 @@ Curl_setup_transfer(
   /* we want header and/or body, if neither then don't do this! */
   if(k->getheader || !data->set.opt_no_body) {
 
-    if(conn->sockfd != CURL_SOCKET_BAD)
+    if(sockindex != -1)
       k->keepon |= KEEP_RECV;
 
-    if(conn->writesockfd != CURL_SOCKET_BAD) {
+    if(writesockindex != -1) {
       struct HTTP *http = data->req.protop;
       /* HTTP 1.1 magic:
 
@@ -2075,7 +2082,7 @@ Curl_setup_transfer(
         /* enable the write bit when we're not waiting for continue */
         k->keepon |= KEEP_SEND;
       }
-    } /* if(conn->writesockfd != CURL_SOCKET_BAD) */
+    } /* if(writesockindex != -1) */
   } /* if(k->getheader || !data->set.opt_no_body) */
 
 }