resolve crash and make more pythonic

[mchugh19]

Experienced scenario

[CRITICAL] Failed to load grains defined in grain file ec2_info.ec2_info in function <function ec2_info at 0x7f8cdfc8bde8>, error:
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/salt/loader.py", line 757, in grains
    ret = funcs[key]()
  File "/var/cache/salt/minion/extmods/grains/ec2_info.py", line 153, in ec2_info
    grains.update(_get_ec2_hostinfo())
  File "/var/cache/salt/minion/extmods/grains/ec2_info.py", line 69, in _get_ec2_hostinfo
    d[_dash_to_snake_case(line[:-1])] = _get_ec2_hostinfo(path + line)
  File "/var/cache/salt/minion/extmods/grains/ec2_info.py", line 69, in _get_ec2_hostinfo
    d[_dash_to_snake_case(line[:-1])] = _get_ec2_hostinfo(path + line)
  File "/var/cache/salt/minion/extmods/grains/ec2_info.py", line 46, in _get_ec2_hostinfo
    if line[-1] != "/":
IndexError: string index out of range

This corrects for that case