pk_wrap: fix: always clear buffer holding private key in eckey_check_pair_psa

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/pk_wrap.c b/library/pk_wrap.c
index 325a0b0..3c14fd1 100644
--- a/library/pk_wrap.c
+++ b/library/pk_wrap.c
@@ -1240,17 +1240,18 @@
     ret = mbedtls_mpi_write_binary(&mbedtls_pk_ec_ro(*prv)->d,
                                    prv_key_buf, curve_bytes);
     if (ret != 0) {
+        mbedtls_platform_zeroize(prv_key_buf, sizeof(prv_key_buf));
         return ret;
     }
 
     status = psa_import_key(&key_attr, prv_key_buf, curve_bytes, &key_id);
+    mbedtls_platform_zeroize(prv_key_buf, sizeof(prv_key_buf));
     ret = PSA_PK_TO_MBEDTLS_ERR(status);
     if (ret != 0) {
         return ret;
     }
 
-    mbedtls_platform_zeroize(prv_key_buf, sizeof(prv_key_buf));
-
+    // From now on prv_key_buf is used to store the public key of prv.
     status = psa_export_public_key(key_id, prv_key_buf, sizeof(prv_key_buf),
                                    &prv_key_len);
     ret = PSA_PK_TO_MBEDTLS_ERR(status);