Fix segmentation fault on missing credentials

[alexey-medvedchikov]

If no credentials supplied stackdriver_exporter binary goes segfaulted:

# ./stackdriver_exporter --google.project-id aproject --monit
oring.metrics-type-prefixes container.googleapis.com --stackdriver.http-timeout=10s
INFO[0000] Starting stackdriver_exporter (version=0.5.1, branch=master, revision=c36707967ebec2fb8977a54003797e5148c24936)  source="stackdriver_exporter.go:122"
INFO[0000] Build context (go=go1.10.5, user=root@857d0832fe06, date=20181119-16:33:41)  source="stackdriver_exporter.go:123"
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x7b5e47]

goroutine 1 [running]:
main.createMonitoringService(0xc420049f38, 0x2, 0x2)
	/go/src/github.com/frodenas/stackdriver_exporter/stackdriver_exporter.go:84 +0xa7
main.main()
	/go/src/github.com/frodenas/stackdriver_exporter/stackdriver_exporter.go:125 +0x24c

The problem was in using googleClient before checking if it's creation succeeded.