Crash if DISPLAY is empty string

[rubenverhack]

xvfbwrapper/xvfbwrapper.py

Line 53 in 118576f

if 'DISPLAY' in os.environ:

Ran into an issue using this library. When os.environ['DISPLAY'] is an empty string, then it will perform the split based on ':' which has no second element. Rather than just checking if DISPLAY is set, it should check if it is set with a correct format.