Fix another memory leak on invalid nid value

In 906cc7e7e9 a memory leak was fixed but
a similar one is present here that needs a fix as well.

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>
(cherry picked from commit e4c5f6d3d9)

src/pki.c

@@ -1160,6 +1160,10 @@ int pki_import_privkey_buffer(enum ssh_keytypes_e type,
                 nid = pki_key_ecdsa_nid_from_name(ssh_string_get_char(i));
                 SSH_STRING_FREE(i);
                 if (nid == -1) {
+                    ssh_string_burn(e);
+                    SSH_STRING_FREE(e);
+                    ssh_string_burn(exp);
+                    SSH_STRING_FREE(exp);
                     goto fail;
                 }