Avoid division by zero if the Swap size is 0

It use to crash with a this error whane there is no Swap:
```
File "/var/lib/shinken/libexec/check_memory_by_ssh.py", line 166, in <module>
  perfdata += ' %s=%s%%;;;0%%;100%%' % (k, int(100 * float(v)/swap_total))   
 ZeroDivisionError: float division by zero
```