Nitz

In function:
static Sint32
IMA_ADPCM_nibble(struct IMA_ADPCM_decodestate *state, Uint8 nybble)
{
 const Sint32 max_audioval = ((1 << (16 - 1)) - 1);
    const Sint32 min_audioval = -(1 << (16 - 1));
    const int index_table[16] = {
        -1, -1, -1, -1,
        2, 4, 6, 8,
        -1, -1, -1, -1,
        2, 4, 6, 8
    };
    const Sint32 step_table[89] = {
        7, 8, 9, 10, 11, 12, 13, 14, 16, 17, 19, 21, 23, 25, 28, 31,
        34, 37, 41, 45, 50, 55, 60, 66, 73, 80, 88, 97, 107, 118, 130,
        143, 157, 173, 190, 209, 230, 253, 279, 307, 337, 371, 408,
        449, 494, 544, 598, 658, 724, 796, 876, 963, 1060, 1166, 1282,
        1411, 1552, 1707, 1878, 2066, 2272, 2499, 2749, 3024, 3327,
        3660, 4026, 4428, 4871, 5358, 5894, 6484, 7132, 7845, 8630,
        9493, 10442, 11487, 12635, 13899, 15289, 16818, 18500, 20350,
        22385, 24623, 27086, 29794, 32767
    };
    Sint32 delta, step;

    /* Compute difference and new sample value */
    step = step_table[state->index];
    // Some Code
}

Here step_table array have the state->index, which might be negative, so it is unsafe to assign this index to step_table array directly.
There would be a check before that to confirm its value.