Correctly handle findMatchingClosedLoopBGReadings loop overrun
Correctly handle findMatchingClosedLoopBGReadings loop overrun

`findMatchingClosedLoopBGReadings` was failing if the last
CLOSED_LOOP_BG_READING event in the history was less than
15 minutes before the end of the history, due to a failure to check for
an array element overrun.

tidepool-org/chrome-uploader,tidepool-org/chrome-uploader,tidepool-org/chrome-uploader