TLS: only reuse connections with the same client cert

CVE-2016-5420
Bug: https://curl.haxx.se/docs/adv_20160803B.html
diff --git a/lib/vtls/vtls.c b/lib/vtls/vtls.c
index 33e209d..3863777 100644
--- a/lib/vtls/vtls.c
+++ b/lib/vtls/vtls.c
@@ -99,6 +99,7 @@
      (data->verifyhost == needle->verifyhost) &&
      safe_strequal(data->CApath, needle->CApath) &&
      safe_strequal(data->CAfile, needle->CAfile) &&
+     safe_strequal(data->clientcert, needle->clientcert) &&
      safe_strequal(data->random_file, needle->random_file) &&
      safe_strequal(data->egdsocket, needle->egdsocket) &&
      safe_strequal(data->cipher_list, needle->cipher_list))