issue: http introspect crashes when provided flow index is not in range

fixed by returning NULL from GetKernelFlowEntry, if index is outside the
range of flow table entries

Change-Id: Ia80b558506a0664e5e2741cbf3d265ab4bea25ca
Closes-bug:#1404833
Merge "issue: http introspect crashes when provided flow index is not in range"