http2: Curl_read should not use the single buffer

... as it does for pipelining when we're multiplexing, as we need the
different buffers to store incoming data correctly for all streams.

lib/sendf.c

@@ -551,7 +551,8 @@ CURLcode Curl_read(struct connectdata *conn, /* connection data */
   ssize_t nread = 0;
   size_t bytesfromsocket = 0;
   char *buffertofill = NULL;
-  bool pipelining = Curl_multi_pipeline_enabled(conn->data->multi);
+  bool pipelining = Curl_multi_pipeline_enabled(conn->data->multi) &&
+    !conn->bits.multiplex;
 
   /* Set 'num' to 0 or 1, depending on which socket that has been sent here.
      If it is the second socket, we set num to 1. Otherwise to 0. This lets