fix missing null check in kdf_test_ctrl

fix missing null check in kdf_test_ctrl

Coverity issue 1453632 noted a missing null check in kdf_test_ctrl
recently.  If a malformed value is passed in from the test file that
does not contain a ':' character, the p variable will be NULL, leading
to a NULL derefence prepare_from_text

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
(Merged from https://github.com/openssl/openssl/pull/23398)