Fix out of bounds errors in Machine.Pop

creature.go

@@ -224,6 +224,10 @@ func (machine *Machine) Push(word int) {
 // Pop pops the last word off of the stack, and returns it, decreasing the stack
 // pointer and reallocating the stack if necessary.
 func (machine *Machine) Pop() (word int) {
+	if machine.pointer <= 0 || machine.pointer >= len(machine.stack) {
+		return
+	}
+	
 	word = machine.stack[machine.pointer]
 	machine.pointer--