fixed divide by zero error when there were invocations but no timed_invocations
fixed divide by zero error when there were invocations but no timed_invocations

(cherry picked from commit 8737e4bbb3d93023784e3b8daf098f5c22fde7d6)

simonzhangsm/voltdb,deerwalk/voltdb,simonzhangsm/voltdb,deerwalk/voltdb,deerwalk/voltdb,deerwalk/voltdb,deerwalk/voltdb,VoltDB/voltdb,VoltDB/voltdb,deerwalk/voltdb,simonzhangsm/voltdb,VoltDB/voltdb,simonzhangsm/voltdb,deerwalk/voltdb,VoltDB/voltdb,simonzhangsm/voltdb,simonzhangsm/voltdb,deerwalk/voltdb,VoltDB/voltdb,VoltDB/voltdb,VoltDB/voltdb,simonzhangsm/voltdb,simonzhangsm/voltdb