fix: correct stack pointer check

Fixes rust-in-action#23

Author: timClicks

ch5/ch5-cpu3/src/main.rs

@@ -42,7 +42,7 @@ struct CPU {
       let sp = self.stack_pointer;
       let stack = &mut self.stack;
 
-      if sp > stack.len() {
+      if sp >= stack.len() {
         panic!("Stack overflow!")
       }
 

ch5/ch5-cpu4/src/main.rs

@@ -81,7 +81,7 @@ impl CPU {
         let sp = self.stack_pointer;
         let stack = &mut self.stack;
 
-        if sp > stack.len() {
+        if sp >= stack.len() {
             panic!("Stack overflow!")
         }