Null pointer dereference issue

[firmianay]

hi, there seems to be a Null pointer dereference issue in the find_env function of the ef-env.c when EF_ENV_USING_CACHE is enable:

del_env(NULL, env, false);

if (find_env(key, &env)) {

size_t key_len = strlen(key);

[armink]

你好，方便提交 PR 吗

[firmianay]

我本身对easyflash不熟悉，只是发现了这个问题，还是作者改一下吧