Fix an incorrect error code addition in pk_parse_key_pkcs8_unencrypted_der

An incorrect error code addition was spotted by the new invasive testing
infrastructure whereby pk_get_pk_alg will always return a high level
error or zero and pk_parse_key_pkcs8_unencrypted_der will try to add
another high level error, resulting in a garbage error code.

Apply the same fix from ae3741e8a to fix the bug.

Signed-off-by: Chris Jones <christopher.jones@arm.com>
diff --git a/ChangeLog.d/fix-pk-parse-key-error-code.txt b/ChangeLog.d/fix-pk-parse-key-error-code.txt
new file mode 100644
index 0000000..3aa330b
--- /dev/null
+++ b/ChangeLog.d/fix-pk-parse-key-error-code.txt
@@ -0,0 +1,2 @@
+Bugfix
+   * Fix an incorrect error code when parsing a PKCS#8 private key.