Check the return code from ASN1_TIME_diff()

The function can fail so we should check the return code.

Found by Coverity
Reviewed-by: NBernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/5339)

apps/ca.c

@@ -1706,7 +1706,9 @@ static int do_body(X509 **xret, EVP_PKEY *pkey, X509 *x509,
 
     if (enddate != NULL) {
         int tdays;
-        ASN1_TIME_diff(&tdays, NULL, NULL, X509_get0_notAfter(ret));
+
+        if (!ASN1_TIME_diff(&tdays, NULL, NULL, X509_get0_notAfter(ret)))
+            goto end;
         days = tdays;
     }