[SPARK-9249] [SPARKR] local variable assigned but may not be used

[[SPARK-9249] local variable assigned but may not be used - ASF JIRA](https://issues.apache.org/jira/browse/SPARK-9249)

https://gist.github.com/yu-iskw/0e5b0253c11769457ea5

Author: Yu ISHIKAWA <yuu.ishikawa@gmail.com>

Closes #7640 from yu-iskw/SPARK-9249 and squashes the following commits:

7a51cab [Yu ISHIKAWA] [SPARK-9249][SparkR] local variable assigned but may not be used