Fix syntax errors in safety_demo.ks

Author: congwang-mk

examples/safety_demo.ks

@@ -117,17 +117,17 @@ fn array_validation_demo(ctx: *xdp_md) -> xdp_action {
   var result: xdp_action = XDP_PASS
 
   // Call safe functions
-  var _ = safe_function(ctx)
-  var _ = bounds_demo(ctx)
-  var _ = moderate_stack_usage(ctx)
+  safe_function(ctx)
+  bounds_demo(ctx)
+  moderate_stack_usage(ctx)
 
   // The following call would trigger warnings:
   // let _ = large_stack_usage(ctx) // Stack overflow warning
 
   // Safe map access
   var key: u32 = 1
   var count = packet_stats[key]
-  if (count != null) {
+  if (count != none) {
     packet_stats[key] = count + 1
   } else {
     packet_stats[key] = 1