Fix: PEM_read_bio_PrivateKey with no-ui / no-stdio

If openssl is compiled with no-ui or no-stdio, then PEM_read_bio_PrivateKey fails if a password but no callback is provided.

The reason is that the premature return in the PEM_def_callback implementation when OPENSSL_NO_STDIO or OPENSSL_NO_UI is defined, comes too early.

This patch moves the ifdef block to the correct place.
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>