Fix crash when 2+ screens are connected, but not all are active when running in X11 mode.

[nphantasm]

When not all connected screens are active XRRGetCrtcInfo returns NULL for the inactive screens resulting in a SIGSEGV when trying to push it to the screens vector.

TODO:

• [x] When user sets --screen-root to a non-existent screen, program still runs without fatal error.
• [x] Maybe this can be checked in the same if statement as the connection check?
  • No. There is a way in XRandr to get only active monitors, but that involves manually creating requests and internal Xorg functions. See this and this
• [x] Is this also a problem on Wayland? No.

[Almamu]

Looks good to me, I'll do a quick test this afternoon (currently 1000am) and merge it. Sorry for the delay, and thanks for the contribution!!