Handle "int_ctx_new:unsupported algorithm" error

Calling EVP_PKEY_CTX_new_id(curve_NID, NULL) causes an error for most
curves that are implemented through the EC low-level API, and in the
last commit we call it for every curve to avoid treating X25519 as a
special case.

Last commit code already handles correctly this failure, but does not
remove these events from the thread error queue, thus some
false-positive warnings are printed at the end of execution.

This commit ensures that the error queue is clean, without flushing
other errors.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1658)