Fix segfault on dock suspend, unplug, resume

drmmode_set_mode can segfault if a USB-C dock with external monitors is disconnected during suspend. It appears that some data structures are not updated, but the associated drmModeConnectorPtr associated with those structures is NULL. Dereferencing that pointer results in Xorg crashing.

Backtrace:

(crtc=crtc@entry=0x55a0c7610390, fb=fb@entry=0x55a0c86d7410, mode=mode@entry=0x55a0c76103a8, x=x@entry=0, y=y@entry=0) at drmmode_display.c:1267
(crtc=0x55a0c7610390, mode=0x55a0c76103a8, rotation=<optimized out>, x=<optimized out>, y=<optimized out>) at drmmode_display.c:1371
(main=main@entry=0x55a0c63f4b40, argc=argc@entry=10, argv=argv@entry=0x7fffb7cefbf8)
at ../sysdeps/nptl/libc_start_call_main.h:58
(main=0x55a0c63f4b40, argc=10, argv=0x7fffb7cefbf8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffb7cefbe8) at ../csu/libc-start.c:381

Fix this by checking the pointer before dereferencing it.

Fixes: drm/amd#2375 (closed) Signed-off-by: Chris Bainbridge chris.bainbridge@gmail.com