Fix build with GCC.

BIO_new_mem_buf takes a void* buf, but internally it never modifies the
buf. It assigns the buffer to another pointer and then marks it as
read-only. So deconsting it should be safe here.

Also fix warning about 'buf' possibly being unused in parse_cert()

Approved by:	bapt
MFC after:	2 days
X-MFC-With:	r257147
Try even harder to find a console before giving up.