Handling zeros in readPerfStat

[iwankgb]

Adds more robust handling for zeros in readPerfStat and fixes #2629 (hopefully).

[iwankgb]

While working on the PR I noticed following linter issues:

ubuntu@malyna4:~/go/src/github.com/google/cadvisor$ golangci-lint run --build-tags libpfm,cgo
WARN [linters context/goanalysis] Failed to get persisted facts: failed to get data from low-level cache by key fact_purity/facts for package yaml: failed to stat file /home/ubuntu/.cache/golangci-lint/4c/4c05768a80d95b176a37aae80ce01dbba0a393ee328bae71015e9bd10d999a71-d: stat /home/ubuntu/.cache/golangci-lint/4c/4c05768a80d95b176a37aae80ce01dbba0a393ee328bae71015e9bd10d999a71-d: no such file or directory
WARN [linters context/goanalysis] Failed to get persisted facts: failed to get data from low-level cache by key ctrlflow/facts for package yaml: failed to stat file /home/ubuntu/.cache/golangci-lint/68/684eaab543416e6ecf05612d3269cb5401637f7c9ab58a9e6d4ff3b84ad99df5-d: stat /home/ubuntu/.cache/golangci-lint/68/684eaab543416e6ecf05612d3269cb5401637f7c9ab58a9e6d4ff3b84ad99df5-d: no such file or directory
integration/tests/api/perf_test.go:50:28: Error return value of `framework.RetryForDuration` is not checked (errcheck)
	framework.RetryForDuration(func() error {
	                          ^
perf/uncore_libpfm.go:302:9: `if` block ends with a `return` statement, so drop this `else` and outdent its block (golint)
	} else {
	       ^
perf/uncore_libpfm.go:101:27: S1019: should use make(uncorePMUs) instead (gosimple)
	pmus := make(uncorePMUs, 0)
	                         ^
perf/uncore_libpfm_test.go:43:80: SA9002: file mode '777' evaluates to 01411; did you mean '0777'? (staticcheck)
	err = ioutil.WriteFile(filepath.Join(firstPMUPath, "cpumask"), []byte("0-1"), 777)
	                                                                              ^
perf/uncore_libpfm_test.go:47:76: SA9002: file mode '777' evaluates to 01411; did you mean '0777'? (staticcheck)
	err = ioutil.WriteFile(filepath.Join(firstPMUPath, "type"), []byte("18"), 777)
	                                                                          ^
perf/uncore_libpfm_test.go:58:81: SA9002: file mode '777' evaluates to 01411; did you mean '0777'? (staticcheck)
	err = ioutil.WriteFile(filepath.Join(secondPMUPath, "cpumask"), []byte("0,1"), 777)
	                                                                               ^
perf/uncore_libpfm_test.go:62:77: SA9002: file mode '777' evaluates to 01411; did you mean '0777'? (staticcheck)
	err = ioutil.WriteFile(filepath.Join(secondPMUPath, "type"), []byte("19"), 777)

I will fix them in separate commit.

[katarzyna-z]

lgtm

[katarzyna-z]

@wacuuu Could you check this fix?

[katarzyna-z]

It seems that the same fix should be provided for uncore perf events, see this line

[wacuuu]

For complicity:
missing the testcase that started whole of the issue:
{ Value: 0, TimeEnabled: 3, TimeRunning: 0, }

[dashpole]

lgtm

[dashpole]

I'll leave this open for a day to let you fix the remaining comments above

[iwankgb]

@dashpole I think it can be merged :)