tls: always reset this.ssl.error after handling

Otherwise assertion may happen:

    src/node_crypto.cc:962: void node::crypto::Connection::ClearError():
    Assertion `handle_->Get(String::New("error"))->BooleanValue() == false'
    failed.

See #5058

lib/tls.js

@@ -906,30 +906,25 @@ SecurePair.prototype.destroy = function() {
 
 
 SecurePair.prototype.error = function() {
+  var err = this.ssl.error;
+  this.ssl.error = null;
+
   if (!this._secureEstablished) {
-    var error = this.ssl.error;
-    if (!error) {
-      error = new Error('socket hang up');
-      error.code = 'ECONNRESET';
+    if (!err) {
+      err = new Error('socket hang up');
+      err.code = 'ECONNRESET';
     }
     this.destroy();
-    this.emit('error', error);
-    return error;
+    this.emit('error', err);
+  } else if (this._isServer &&
+             this._rejectUnauthorized &&
+             /peer did not return a certificate/.test(err.message)) {
+    // Not really an error.
+    this.destroy();
   } else {
-    var err = this.ssl.error;
-    this.ssl.error = null;
-
-    if (this._isServer &&
-        this._rejectUnauthorized &&
-        /peer did not return a certificate/.test(err.message)) {
-      // Not really an error.
-      this.destroy();
-    } else {
-      this.cleartext.emit('error', err);
-    }
-
-    return err;
+    this.cleartext.emit('error', err);
   }
+  return err;
 };
 
 // TODO: support anonymous (nocert) and PSK