diag: Fix possible buffer overflow

Currently, while updating the real time vote
there is a possibility that index will go beyond
the array size which will lead to buffer overflow.
This patch rectifies the index checking to avoid
possible buffer overflow.

Change-Id: I71fa3698eacdcb2933dc388ca8571bb6f952becc
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>