Fix the ocsp code to actually check for errors when comparing time values

Fix the ocsp code to actually check for errors when comparing time values
which was not being done due to a lack of checking of the return code for
X509_cmp_time.  Ensure that we only compare GERNERALIZEDTIME values because
this is what is specified by RFC6960.

Issue reported, and fix provided by Kazuki Yamaguchi <k@rhe.jp>
ok bcook@