output: check wlr_output in repaint handler
output: check wlr_output in repaint handler

It's possible for the output to be disconnected in just the right moment
for wlr_output to be NULL in the repaint handler, causing a crash. This
check fixes that crash.

1ace/sway,SirCmpwn/sway,1ace/sway,ascent12/sway,ascent12/sway,ascent12/sway,1ace/sway