Fixed NullPointerException w/ virtual displays

[abc123me]

Fixed a NullPointerException caused by trying to call getAvailableDisplayModes
when the user has a virtual display.
LWJGL tries to get the bits per pixel of the display, nGetAvailableDisplayModes returns null,
then the length check causes a NullPointerException on line 950.

And yes, I know LWJGL2 is legacy but LibGDX still uses it and I like LibGDX so i'm not switching to LWJGL3!

Edit:
This may be related to issue #129 as well as this issue, this SKLauncher crash, and this random pastebin

xrandr: ```
Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 32767 x 32767
LVDS1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 310mm x 170mm
1920x1080 60.00*+ 59.93
1680x1050 59.88
1600x1024 60.17
1400x1050 59.98
1600x900 60.00
1280x1024 60.02
1440x900 59.89
1280x960 60.00
1368x768 60.00
1360x768 59.80 59.96
1152x864 60.00
1280x720 60.00
1024x768 60.00
1024x576 60.00
960x540 60.00
800x600 60.32 56.25
864x486 60.00
640x480 59.94
720x405 60.00
640x360 60.00
DP1 disconnected (normal left inverted right x axis y axis)
DP2 disconnected (normal left inverted right x axis y axis)
DP3 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
HDMI2 disconnected (normal left inverted right x axis y axis)
HDMI3 disconnected (normal left inverted right x axis y axis)
VGA1 disconnected 1920x1080+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
VIRTUAL1 disconnected (normal left inverted right x axis y axis)
1920x1080 (0x4d) 148.500MHz +HSync +VSync
h: width 1920 start 2008 end 2052 total 2200 skew 0 clock 67.50KHz
v: height 1080 start 1084 end 1089 total 1125 clock 60.00Hz