Do not dereference getPortStats gRPC message pointer on error

[pudelkoM]

Currently pfcpagent will crash when the getPortStats gRPC call fails for any reason, because we dereference a nil pointer to the response message.

From the docs:

If the call doesn’t return an error, then we can read the response information from the server from the first return value.

https://grpc.io/docs/languages/go/basics/#calling-service-methods

Logs:

time="2021-11-06T17:53:03Z" level=info msg="Error calling get_summary on module access_measure rpc error: code = Unavailable desc = error reading from server: EOF" func="main.(*bess).measure" file="/pfcpiface/bess.go:219"
time="2021-11-06T17:53:03Z" level=info msg="Error calling get_summary on module core_measure rpc error: code = Unavailable desc = connection closed before server preface received" func="main.(*bess).measure" file="/pfcpiface/bess.go:219"
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0x99f567]

goroutine 496605 [running]:
main.(*bess).getPortStats(0xc000340050, {0xc0001b9798, 0x6})
	/pfcpiface/bess.go:242 +0x147
main.(*bess).portStats.func1({0xb5430e, 0x4}, {0xc0001b9798, 0x4})
	/pfcpiface/bess.go:279 +0x10d
main.(*bess).portStats(0xc000952f20, 0xc00032c2a0, 0x861f3f)
	/pfcpiface/bess.go:294 +0x69
main.(*upfCollector).portStats(...)
	/pfcpiface/telemetry.go:117
main.(*upfCollector).Collect(0xc00032c2a0, 0xc000952f60)
	/pfcpiface/telemetry.go:112 +0x65
github.com/prometheus/client_golang/prometheus.(*Registry).Gather.func1()