[base] PerformanceCounter: Fixes bug in access out-of-bound data.

Following caused by a request to /server-status (status plugin) as the very
first request to the service.

==29198== Conditional jump or move depends on uninitialised value(s)
==29198==    at 0x8F6FB6A: x0::PerformanceCounter<900u, double>::average(unsigned int) const (PerformanceCounter.h:112)
==29198==    by 0x8F6CD9F: x0::HttpWorker::fetchPerformanceCounts(double*, double*, double*) const (HttpWorker.h:288)
==29198==    by 0x8F6E816: StatusPlugin::createResponseBody(bool) (status.cpp:257)
==29198==    by 0x8F6E5EF: StatusPlugin::handleRequest(x0::HttpRequest*, x0::FlowArray const&) (status.cpp:227)
==29198==    by 0x8F71143: void x0::HttpPlugin::handler_thunk<StatusPlugin, &(StatusPlugin::handleRequest(x0::HttpRequest*, x0::FlowArr
ay const&))>(void*, x0::FlowArray&, void*) (HttpPlugin.h:156)
==29198==    by 0x55C0F8D: x0::FlowBackend::Callback::invoke(int, x0::FlowValue*, void*) (in /home/trapni/projects/x0/lib/libXzeroFlow.
so.0.6.0)
==29198==    by 0x55C0BF1: x0::FlowBackend::invoke(int, int, x0::FlowValue*, void*) (FlowBackend.cpp:75)
==29198==    by 0x55C0E34: flow_backend_callback (FlowBackend.cpp:127)
==29198==    by 0x40525B9: ???
==29198==    by 0x514747C: x0::HttpWorker::handleRequest(x0::HttpRequest*) (HttpWorker.cpp:269)
==29198==    by 0x513FD13: x0::HttpConnection::onMessageHeaderEnd() (HttpConnection.cpp:362)
==29198==    by 0x5178655: x0::HttpMessageProcessor::process(x0::BufferRef const&, unsigned long*) (HttpMessageProcessor.cpp:893)