Check eth intf existence before creating VLAN

Request to create VLAN interface for non-existing interface causes
non-existing ethernet-interface object to be accessed which in turn
causes segmentation fault.Check for requested ethernet root intf and
allow the creation of VLAN interface for ethernet objects.

Tested:
 POSTMAN Request & Response
  Request: https://xx.xx.xx.xx/redfish/v1/Managers/bmc/EthernetInterfaces/abcd/VLANs
  Method: POST
  Body Parameters: {"VLANId": 1, "VLANEnable": true }
  Response:
   "error": {
         "@Message.ExtendedInfo": [
             {
                "@odata.type": "#Message.v1_1_1.Message",
                "Message": "The request failed due to an internal service error.  The service is still operational.",
                "MessageArgs": [],
                "MessageId": "Base.1.8.1.InternalError",
                "MessageSeverity": "Critical",
                "Resolution": "Resubmit the request.  If the problem persists, consider resetting the service."
             }
         ],
         "code": "Base.1.8.1.InternalError",
         "message": "The request failed due to an internal service error.  The service is still operational."
       }

Signed-off-by: sureshvijayv1 <suresh.vijayakumar@intel.com>
Change-Id: I38e5cf64935e120a14b6b91c631e7e20b58789a1
diff --git a/src/network_manager.cpp b/src/network_manager.cpp
index fe59f0b..8b4c5c2 100644
--- a/src/network_manager.cpp
+++ b/src/network_manager.cpp
@@ -188,6 +188,11 @@
 
 ObjectPath Manager::vlan(IntfName interfaceName, uint32_t id)
 {
+    if (!hasInterface(interfaceName))
+    {
+        elog<ResourceNotFound>();
+    }
+
     return interfaces[interfaceName]->createVLAN(id);
 }