libdict dict_del function logic flaw

[YvesCjl]

To reproduce:

char s[105];
int main()
{
dict *d = dict_new();
read( 0, s, 100 ); // input 100 zeros, i.e., '00000000'...
dict_del( d, s ); // successful, now d->used==-1
return 0;
}

By the way, dict_free(d) doesn't check if dict is valid, may lead to heap crash. To reproduce:

int main()
{
dict *d = dict_new();
dict_free( d );
dict_free( d ); // heap crash
return 0;
}

[gozfree]

fix patch https://gitee.com/gozfreee/gear-lib/commit/02db43e0d0d76a315dd5d58298bc4b9102539d7a

double dict_free no need fix, just like malloc -> free -> free will also cause heap crash, should be make sure by users