[SPARK-19857][YARN] Correctly calculate next credential update time.

Add parentheses so that both lines form a single statement; also add
a log message so that the issue becomes more explicit if it shows up
again.

Tested manually with integration test that exercises the feature.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #17198 from vanzin/SPARK-19857.

(cherry picked from commit 8e41c2ee)
Signed-off-by: Marcelo Vanzin <vanzin@cloudera.com>