Mark usage, fatal, and panic as _X_NORETURN

Mark usage, fatal, and panic as _X_NORETURN

xrandr.c:634:13: warning: Access to field 'next' results in a dereference of a null pointer (loaded from variable 'output')
    output->next = NULL;
    ~~~~~~  ^
xrandr.c:1214:6: warning: Access to field 'mode' results in a dereference of a null pointer (loaded from variable 'crtc_info')
        if (crtc_info->mode == None)
            ^~~~~~~~~
xrandr.c:1252:5: warning: Array access (via field 'outputs') results in a null pointer dereference
    crtc->outputs[crtc->noutput++] = output;
    ^     ~~~~~~~
xrandr.c:1638:33: warning: Access to field 'name' results in a dereference of a null pointer (loaded from variable 'output_info')
        set_name_string (&output_name, output_info->name);
                                       ^~~~~~~~~~~
xrandr.c:1725:10: warning: Access to field 'changes' results in a dereference of a null pointer (loaded from variable 'output')
            if (output->changes)
                ^~~~~~
xrandr.c:1848:10: warning: Access to field 'mode_info' results in a dereference of a null pointer (loaded from variable 'relation')
            if (relation->mode_info == NULL)
                ^~~~~~~~
xrandr.c:3194:11: warning: Array access (from variable 'mode_shown') results in a null pointer dereference
                    if (mode_shown[j]) continue;
                        ^~~~~~~~~~

Found-by: clang static analyzer
Signed-off-by: Jeremy Huddleston <jeremyhu@apple.com>