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

@@ -906,20 +906,17 @@ SecurePair.prototype.destroy = function() {
 
 
 SecurePair.prototype.error = function() {
-  if (!this._secureEstablished) {
-    var error = this.ssl.error;
-    if (!error) {
-      error = new Error('socket hang up');
-      error.code = 'ECONNRESET';
-    }
-    this.destroy();
-    this.emit('error', error);
-    return error;
-  } else {
   var err = this.ssl.error;
   this.ssl.error = null;
 
-    if (this._isServer &&
+  if (!this._secureEstablished) {
+    if (!err) {
+      err = new Error('socket hang up');
+      err.code = 'ECONNRESET';
+    }
+    this.destroy();
+    this.emit('error', err);
+  } else if (this._isServer &&
              this._rejectUnauthorized &&
              /peer did not return a certificate/.test(err.message)) {
     // Not really an error.
@@ -927,9 +924,7 @@ SecurePair.prototype.error = function() {
   } else {
     this.cleartext.emit('error', err);
   }
-
   return err;
-  }
 };
 
 // TODO: support anonymous (nocert) and PSK