nss: fix tight loop in non-blocking TLS handhsake over proxy

... in case the handshake completes before entering
CURLM_STATE_PROTOCONNECT

Bug: https://bugzilla.redhat.com/1388162

RELEASE-NOTES

@@ -56,6 +56,7 @@ This release includes the following bugfixes:
  o INSTALL: converted to markdown => INSTALL.md
  o curl_multi_remove_handle: fix a double-free [25]
  o multi: fix inifinte loop in curl_multi_cleanup() [26]
+ o nss: fix tight loop in non-blocking TLS handhsake over proxy [27]
 
 This release includes the following known bugs:
 
@@ -104,3 +105,4 @@ References to bug reports and discussions on issues:
  [24] = https://curl.haxx.se/bug/?i=1069
  [25] = https://curl.haxx.se/bug/?i=1083
  [26] = https://curl.haxx.se/mail/lib-2016-10/0011.html
+ [27] = https://bugzilla.redhat.com/1388162

lib/vtls/nss.c

@@ -1921,8 +1921,11 @@ static CURLcode nss_connect_common(struct connectdata *conn, int sockindex,
   const bool blocking = (done == NULL);
   CURLcode result;
 
-  if(connssl->state == ssl_connection_complete)
+  if(connssl->state == ssl_connection_complete) {
+    if(!blocking)
+      *done = TRUE;
     return CURLE_OK;
+  }
 
   if(connssl->connecting_state == ssl_connect_1) {
     result = nss_setup_connect(conn, sockindex);