cert_stuff: avoid double free in the PKCS12 code

In the pkcs12 code, we get a list of x509 records returned from
PKCS12_parse but when iterating over the list and passing each to
SSL_CTX_add_extra_chain_cert() we didn't also properly remove them from
the "stack", which made them get freed twice (both in sk_X509_pop_free()
and then later in SSL_CTX_free).

This isn't really documented anywhere...

Bug: http://curl.haxx.se/bug/view.cgi?id=1236
Reported-by: Nikaiw