multi_done: if multiplexed, make conn->data point to another transfer

... since the current transfer is being killed. Setting to NULL is
wrong, leaving it pointing to 'data' is wrong since that handle might be
about to get freed.

Fixes #4845
Closes #4858
Reported-by: dmitrmax on github

1 files changed, 2 insertions, 0 deletions

diff --git a/lib/url.c b/lib/url.c
index 689668e04..837e62a09 100644
--- a/lib/url.c
+++ b/lib/url.c
@@ -1213,6 +1213,8 @@ ConnectionExists(struct Curl_easy *data,
         }
       }
 
+      DEBUGASSERT(!check->data || GOOD_EASY_HANDLE(check->data));
+
       if(!canmultiplex && check->data)
         /* this request can't be multiplexed but the checked connection is
            already in use so we skip it */