CompactStorage fails and terminates agent will panic

[ardias]

CompactStorage at the start of the agent can make the agent panic when walking the storeage dir:

time="2020-09-08T19:01:28+02:00" level=fatal msg="runtime error: invalid memory address or nil pointer dereference" stacktrace="goroutine 1 [running]:\nruntime/debug.Stack(0xc0000c46a8, 0xce59c0, 0x15ce320) /usr/local/go/src/runtime/debug/stack.go:24 +0x9d\ngithub.com/newrelic/infrastructure-agent/pkg/helpers/recover.PanicHandler(0x0) /go/src/github.com/newrelic/infrastructure-agent/pkg/helpers/recover/recover.go:30 +0x51\npanic(0xce59c0, 0x15ce320) /usr/local/go/src/runtime/panic.go:975 +0x3e3\ngithub.com/newrelic/infrastructure-agent/internal/agent/delta.(*Store).StorageSize.func1(0xc02a265000, 0x7c, 0x0, 0x0, 0x1031c20, 0xc0239a69f0, 0x0, 0x0) /go/src/github.com/newrelic/infrastructure-agent/internal/agent/delta/store.go:272 +0x2b\npath/filepath.walk(0xc02a26c620, 0x62, 0x1049b40, 0xc02a2781a0, 0xc0000c4b00, 0x0, 0x0) /usr/local/go/src/path/filepath/path.go:380 +0x20c\npath/filepath.walk(0xc028524780, 0x33, 0x1049b40, 0xc02807e820, 0xc0000c4b00, 0x0, 0x0) /usr/local/go/src/path/filepath/path.go:384 +0x2ff\npath/filepath.walk(0xc000799aa0, 0x27, 0x1049b40, 0xc02807e270, 0xc0000c4b00, 0x0, 0xc022105ae0) /usr/local/go/src/path/filepath/path.go:384 +0x2ff\npath/filepath.Walk(0xc000799aa0, 0x27, 0xc0000c4b00, 0xc0000c4b08, 0xc000ceaf30) /usr/local/go/src/path/filepath/path.go:406 +0xff\ngithub.com/newrelic/infrastructure-agent/internal/agent/delta.(*Store).StorageSize(0xc00047d020, 0xc000799aa0, 0x27, 0xc00417e840, 0xc01c7b9740, 0xc0000c4abd) /go/src/github.com/newrelic/infrastructure-agent/internal/agent/delta/store.go:271 +0x66\ngithub.com/newrelic/infrastructure-agent/internal/agent/delta.(*Store).CompactStorage(0xc00047d020, 0xc000ceaf30, 0x2f, 0x1400000, 0x0, 0x0) /go/src/github.com/newrelic/infrastructure-agent/internal/agent/delta/store.go:243 +0x41\ngithub.com/newrelic/infrastructure-agent/internal/agent.(*patchSenderIngest).Process(0xc00417e840, 0x0, 0x0) /go/src/github.com/newrelic/infrastructure-agent/internal/agent/patch_sender.go:162 +0x50f\ngithub.com/newrelic/infrastructure-agent/internal/agent.(*Agent).sendInventory(0xc000184800, 0xc00016ceb0) /go/src/github.com/newrelic/infrastructure-agent/internal/agent/agent.go:942 +0xbe\ngithub.com/newrelic/infrastructure-agent/internal/agent.(*Agent).Run(0xc000184800, 0x0, 0x0) /go/src/github.com/newrelic/infrastructure-agent/internal/agent/agent.go:920 +0x9f1\nmain.initializeAgentAndRun(0xc000426300, 0x0, 0xc000470150, 0x2a, 0xc0004b83c0, 0x1d, 0xc0001edc80, 0x34, 0xc0004700f0, 0x28, ...) /go/src/github.com/newrelic/infrastructure-agent/cmd/newrelic-infra/newrelic-infra.go:323 +0x1cb7\nmain.main() /go/src/github.com/newrelic/infrastructure-agent/cmd/newrelic-infra/newrelic-infra.go:165 +0x80b\n"

[varas]

Fixed at #102