Fix overflow when the scope is pop while the index is 0 and add a tes…

…t for it

src/evaluation/evaluator.rs

@@ -204,4 +204,13 @@ mod tests {
         brainfuck.evaluate(None, Some(false));
         assert_eq!(String::from_utf8(brainfuck.output_buffer).unwrap(), "AA");
     }
+
+    #[test]
+    fn test_overflow_scope_pop() {
+        let program = String::from("{´}=");
+        let mut parser = ast::Parser::new(program);
+        let instructions = parser.parse();
+        let mut evaluator = Evaluator::new(instructions);
+        evaluator.evaluate(None, Some(false));
+    }
 }

src/evaluation/scope.rs

@@ -136,7 +136,9 @@ where
 
     pub fn pop(&mut self) {
         self.scopes.pop();
-        self.scope_index -= 1;
+        if self.scope_index > 0 {
+            self.scope_index -= 1;
+        }
     }
 }