Fix for intermittent exception while parsing Xrandr modes

[cormacc]

Observed that Xrandr occasionally appends a few lines to the output, resulting in a runtime exception during mode parsing.
Can't reproduce the condition reliably, but seems to have something to do with disconnection/reconnection. Once I do get into this state, the exception is repeatable.

Excerpt below from last time the condition occurred (see final 3 lines):

DP-4 connected 3840x2160+0+0 (normal left inverted right x axis y axis)
600mm x 340mm
3840x2160 60.00*+ 30.00
2560x1440 59.95
1920x1080 60.00 59.94
1600x900 60.00
1280x1024 60.02
1280x800 59.81
1280x720 60.00 59.94
1152x864 59.96
1024x768 60.00
800x600 60.32
720x480 59.94
640x480 59.94 59.93
3840x2160 (0x2ca) 297.000MHz +HSync +VSync
h: width 3840 start 4016 end 4104 total 4400 skew 0 clock 67.50KHz
v: height 2160 start 2168 end 2178 total 2250 clock 30.00Hz

This is probably obscure enough to ignore, but think the fix has no negative implications. The original implementation had a smaller footprint (about 3 lines) but involved applying the regex twice, which offended my inner anal-retentive (I mostly do embedded C these days).

The issue occurs intermittently running arch linux on a 2013 macbook pro running nvidia drivers 375.20, libxrandr 1.5.1 with a 27" LG external display