Update an error reason code to be ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED

The most likely explanation for us ending up at this point in the code
is that we were called by the user application incorrectly - so use an
appropriate error code.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4778)