Convert a GF(2^m) polynomial BIGNUM into a descending exponent index array.
Number of exponents written (excluding -1), or 0 if max is too small / on error.
| Name | Description |
|---|---|
| a | Polynomial whose set bits become exponents. |
| p | Destination array of size at least max; filled then terminated with -1. |
| max | Capacity of p including room for the -1 terminator. |