Prevent null-pointer deference during vim_eval #785

If the eval_expr call in vim_eval returns NULL, a null-pointer deference
would happen a few frames down, in vim_to_object_rec

src/nvim/api/vim.c

@@ -61,6 +61,10 @@ Object vim_eval(String str, Error *err)
   try_start();
   typval_T *expr_result = eval_expr((char_u *) str.data, NULL);
 
+  if (!expr_result) {
+    set_api_error("Failed to eval expression", err);
+  }
+
   if (!try_end(err)) {
     // No errors, convert the result
     rv = vim_to_object(expr_result);