MdeModulePkg/NetworkPkg: Checking for NULL pointer before use.

Contributed-under: TianoCore Contribution Agreement 1.1

Signed-off-by: Sivaraman Nainar <sivaramann@amiindia.co.in>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>

NetworkPkg/HttpBootDxe/HttpBootConfig.c

@@ -473,6 +473,9 @@ HttpBootFormCallback (
     // Get user input URI string
     //
     Uri = HiiGetString (CallbackInfo->RegisteredHandle, Value->string, NULL);
+    if(Uri == NULL) {
+    	return EFI_INVALID_PARAMETER;
+    }        
 
     //
     // The URI should be either an empty string (for corporate environment) ,or http(s) for home environment.