Don't create an ECX key with short keys

Don't create an ECX key with short keys

If an ECX key is created and the private key is too short, a fromdata
call would create the key, and then later detect the error and report it
after freeing the key. However freeing the key was calling
OPENSSL_secure_clear_free() and assuming that the private key was of the
correct length. If it was actually too short this will write over memory
that it shouldn't.

Fixes #17017

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17041)