Fix Coverity issue in kdf_snmpkdf_set_ctx_params

Fix Coverity issue in kdf_snmpkdf_set_ctx_params

Coverity flagged a use before NULL check error in
kdf_snmpkdf_set_ctx_params.

In this function the ctx pointer was dereferenced to obtain a
OSSL_LIB_CTX pointer, and only after that dereference was ctx checked
for being NULL.

fix is pretty clear, just move the OSSL_LIB_CTX computation down to a
point after ctx is checked for NULL.

fixes openssl/project#1765

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Norbert Pocs <norbertp@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/29400)