The const conundrum in OpenSSL

In OpenSSL, some functions require the use of const but then down the road others require me to not use const for the same variable. An example is when I create a new BIGNUM I need to use const but then to free it I cannot have const.

Is there a trick to solve this? What would be the proper why to get rid of the compiler warnings?

Function signatures:

const BIGNUM *EC_KEY_get0_private_key(const EC_KEY *key);
void BN_free(BIGNUM *a);

Offending code:

EC_KEY *ec_key = ...;
/*const*/ BIGNUM *bignum = NULL;
bignum = EC_KEY_get0_private_key(ec_key);
BN_free(bignum);

Compiler warnings:

------------Without const in bignum------------:
warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  bignum = EC_KEY_get0_private_key(ec_key);
     ^

------------With const in bignum------------:
warning: passing argument 1 of ‘BN_free’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  BN_free(bignum);
          ^
In file included from /usr/include/openssl/asn1.h:74:0,
                 from /usr/include/openssl/objects.h:965,
                 from /usr/include/openssl/evp.h:94,
                 from ecdh.c:5:
/usr/include/openssl/bn.h:493:6: note: expected ‘BIGNUM * {aka struct bignum_st *}’ but argument is of type ‘const BIGNUM * {aka const struct bignum_st *}’
 void BN_free(BIGNUM *a);

Edit to add working example:

#include <openssl/evp.h>
#include <openssl/ec.h>
#include <openssl/bn.h>

int main()
{
    EC_KEY *ec_key = NULL;
    const BIGNUM *bignum = NULL;

    ec_key = EC_KEY_new_by_curve_name(NID_X9_62_prime256v1);
    bignum = EC_KEY_get0_private_key(ec_key);

    //BN_free(bignum);    // unnecessary
    EC_KEY_free(ec_key);
    return 0;
}
//$ gcc -g -Wall -Wextra -o leak_ecdh leak_ecdh.c -lcrypto && valgrind --leak-check=yes ./leak_ecdh
//==26202== HEAP SUMMARY:
//==26202==     in use at exit: 0 bytes in 0 blocks
//==26202==   total heap usage: 104 allocs, 104 frees, 4,220 bytes allocated
//==26202== 
//==26202== All heap blocks were freed -- no leaks are possible