g10: Simplify semantics of get_pubkey_byname.

* g10/getkey.c (get_pubkey_byname): If R_KEYBLOCK is not NULL, return
the keyblock in R_KEYBLOCK independent of whether PK is set or not.

Currently, no caller invokes get_pubkey_byname with PK==NULL and
R_KEYBLOCK != NULL. Thus, this change does not change any behavior.

• Signed-off-by: Neal H. Walfield <neal@g10code.com>.