Fix potential memory leak in ASN1_TIME_to_generalizedtime()

If ret is allocated, it may be leaked on error.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2666)
(cherry picked from commit 4483e234)