Agent socket errors bubble up to req only if req exists

Fixes #836.

lib/http.js

@@ -1194,13 +1194,12 @@ Agent.prototype._establishNewConnection = function() {
       req = self.queue.shift();
       assert(req._queue === self.queue);
       req._queue = null;
-    } else {
-      // No requests on queue? Where is the request
-      assert(0);
     }
 
-    req.emit('error', err);
-    req._hadError = true; // hacky
+    if (req) {
+      req.emit('error', err);
+      req._hadError = true; // hacky
+    }
 
     // clean up so that agent can handle new requests
     parser.finish();