Don't crash on storage if redis errors
Don't crash on storage if redis errors

Found by running tests when redis isn't running. On top of checking for
an error, which we weren't doing properly, we need a bounds check. Stack
trace below.

	panic: runtime error: index out of range

	goroutine 6889 [running]:
	github.com/TykTechnologies/tyk.(*RedisStorageManager).SetRollingWindow(0xc420269400, 0xc42026c4e0, 0xc, 0x3c, 0xe2ab80, 0x2, 0x0, 0x0, 0x0, 0x0)
		/home/mvdan/go/src/github.com/TykTechnologies/tyk/storage_handlers.go:769 +0xee7
	created by github.com/TykTechnologies/tyk.(*DefaultHealthChecker).StoreCounterVal
		/home/mvdan/go/src/github.com/TykTechnologies/tyk/api_healthcheck.go:78 +0x490

lonelycode/tyk,nebolsin/tyk,mvdan/tyk,mvdan/tyk,nebolsin/tyk,mvdan/tyk,mvdan/tyk,nebolsin/tyk,nebolsin/tyk,nebolsin/tyk,mvdan/tyk,nebolsin/tyk,nebolsin/tyk,mvdan/tyk,mvdan/tyk,lonelycode/tyk,nebolsin/tyk,lonelycode/tyk,mvdan/tyk