Fix max call depth and add test.

tests/stack.rs

@@ -0,0 +1,29 @@
+use rhai::{Engine, EvalAltResult};
+
+#[test]
+fn test_stack_overflow() -> Result<(), EvalAltResult> {
+    let engine = Engine::new();
+
+    assert_eq!(
+        engine.eval::<i64>(
+            r"
+                fn foo(n) { if n == 0 { 0 } else { n + foo(n-1) } }
+                foo(38)
+    ",
+        )?,
+        741
+    );
+
+    match engine.eval::<()>(
+        r"
+            fn foo(n) { if n == 0 { 0 } else { n + foo(n-1) } }
+            foo(1000)
+    ",
+    ) {
+        Ok(_) => panic!("should be stack overflow"),
+        Err(EvalAltResult::ErrorStackOverflow(_)) => (),
+        Err(_) => panic!("should be stack overflow"),
+    }
+
+    Ok(())
+}