x509_vfy: remove redundant stack allocation

x509_vfy: remove redundant stack allocation

Fix CID 1472833 by removing a codepath that attempts to allocate a
stack if not already allocated, when the stack was already allocated
unconditionally a few lines previously.

Interestingly enough, this additional allocation path (and the comment
describing the need for it) were added in commit
69664d6af0cdd7738f55d10fbbe46cdf15f72e0e, also prompted by Coverity(!).
It seems that the intervening (and much more recent) commit
d53b437f9992f974c1623e9b9b9bdf053aefbcc3 that allowed sk_X509_dup()
to accept a NULL argument allowed the earlier initialization path
to unconditionally allocate a stack, rendering this later allocation fully
redundant.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/14161)