Fix double-free bugs in EC group precomputation state

Reviewed-by: NRich Salz <rsalz@openssl.org>

crypto/ec/ecp_nistp224.c

@@ -1560,6 +1560,7 @@ int ec_GFp_nistp224_precompute_mult(EC_GROUP *group, BN_CTX *ctx)
 
     /* throw away old precomputation */
     EC_nistp224_pre_comp_free(group->pre_comp.nistp224);
+    group->pre_comp.nistp224 = NULL;
     if (ctx == NULL)
         if ((ctx = new_ctx = BN_CTX_new()) == NULL)
             return 0;

crypto/ec/ecp_nistp256.c

@@ -2185,6 +2185,7 @@ int ec_GFp_nistp256_precompute_mult(EC_GROUP *group, BN_CTX *ctx)
 
     /* throw away old precomputation */
     EC_nistp256_pre_comp_free(group->pre_comp.nistp256);
+    group->pre_comp.nistp256 = NULL;
     if (ctx == NULL)
         if ((ctx = new_ctx = BN_CTX_new()) == NULL)
             return 0;

crypto/ec/ecp_nistp521.c

@@ -2009,6 +2009,7 @@ int ec_GFp_nistp521_precompute_mult(EC_GROUP *group, BN_CTX *ctx)
 
     /* throw away old precomputation */
     EC_nistp521_pre_comp_free(group->pre_comp.nistp521);
+    group->pre_comp.nistp521 = NULL;
     if (ctx == NULL)
         if ((ctx = new_ctx = BN_CTX_new()) == NULL)
             return 0;

crypto/ec/ecp_nistz256.c

@@ -770,6 +770,7 @@ __owur static int ecp_nistz256_mult_precompute(EC_GROUP *group, BN_CTX *ctx)
 
     /* if there is an old NISTZ256_PRE_COMP object, throw it away */
     EC_nistz256_pre_comp_free(group->pre_comp.nistz256);
+    group->pre_comp.nistz256 = NULL;
     generator = EC_GROUP_get0_generator(group);
     if (generator == NULL) {
         ECerr(EC_F_ECP_NISTZ256_MULT_PRECOMPUTE, EC_R_UNDEFINED_GENERATOR);