Incomplete data hashing

[R0flan4k]

it's possible to invade stack data without error message.

Steps to reproduce
In task5.cpp:

    for(i = 0; i < 6; i++)
    {
        stack_push(&stk, i + 10);
    }

    stk.data[5] = 0xAB0BA;

    for(; i < 20; i++)
    {
        stack_push(&stk, i + 10);
    }

Expected Behaviour
An error was expected after changing the value of stk.data[5].

Current Behaviour
stack_is_invalid() doesn't detect changes.

Possible solution
oat_hash(stk->data, (size_t) stk->capacity) doesn't fully hash the data. Expected function call:

    oat_hash(stk->data, (size_t) stk->capacity * sizeof(elem_t));

Version: commit 5d29b89