Ensure verify error is set when X509_verify_cert() fails

Set ctx->error = X509_V_ERR_OUT_OF_MEM when verificaiton cannot
continue due to malloc failure.  Also, when X509_verify_cert()
returns <= 0 make sure that the verification status does not remain
X509_V_OK, as a last resort set it it to X509_V_ERR_UNSPECIFIED,
just in case some code path returns an error without setting an
appropriate value of ctx->error.

Reviewed-by: Richard Levitte <levitte@openssl.org>