test/evp_extra_test.c: fix null pointer dereference

It's actually not a real issue but caused by the absence of the default case
which does not occur in reality but which makes coverity see a code path where
pkey remains unassigned.

Reported by Coverity Scan (CID 1423323)
[extended tests]

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7158)