Check indexes etc for `at`

src/object.h

@@ -40,7 +40,8 @@ enum errorCode {
     NOT_A_LIST,
     SCRIPT_NOT_FOUND,
     NO_CLONE_SPECIFIED,
-    CAN_ONLY_EVAL_STRINGS
+    CAN_ONLY_EVAL_STRINGS,
+    INDEX_PAST_END,
 };
 
 #define MALLOC_FLAG 64

src/pebblisp.c

@@ -462,7 +462,12 @@ Object prepend(Object list, Object newElement, struct Environment *env)
 
 Object at(Object index, Object list, struct Environment *env)
 {
-    return cloneObject(*itemAt(&list, index.number));
+    const Object *found = itemAt(&list, index.number);
+    if(found) {
+        return cloneObject(*found);
+    } else {
+        return errorObject(INDEX_PAST_END);
+    }
 }
 
 Object rest(Object list, Object ignore, struct Environment *env)