OpenSSL: Fix crypto_bignum_to_bin() with padlen == 0

OpenSSL: Fix crypto_bignum_to_bin() with padlen == 0

The earlier change to add support for BN_bn2binpad() and
BN_bn2bin_padded() broke this function for cases where no padding is
used (padlen == 0). Those would have always failed after the changes and
the function would return -1. There are no such cases in the current
hostap.git, so this did not have any real issues, but anyway, better fix
this function to match its documentation.

Fixes: 1e237903f5b5 ("OpenSSL: Use BN_bn2binpad() or BN_bn2bin_padded() if available")
Signed-off-by: Jouni Malinen <j@w1.fi>