Decrement ref counter only once on key object free

[pugo]

This is a suggested solution to the issue I describe in issue #527.

By moving the decrement of obj->refcnt to after the if-section for obj->evp_key the counter will not be decremented twice if obj->evp_key is true and we get another call as result of the EVP_PKEY_free().

Without this patch the decrement will be done twice and the comparison with 0 will fail since the value goes negative. This means that the cleanup code at the end is never run.

As far as I can see there shouldn't be scenarios where the current behavior is wanted, but I of course do not have the big picture as clear as you have.