shared memory identifier could be zero

[azmagillian]

Hi,
As far as i understand shared memory identifier could be zero so i think this check should change and check for a value of -1 to return "Cannot read shared memory segment: SHMID not set"
I could do a pull request if you agree!

shmtool/shm/shm.go

Line 117 in d9c620f

if self.Id == 0 {

[ghetzel]

Absolutely! A PR would be fantastic, thank you :)