main: reject --xwayland-count below 1

[adamsjo95]

--xwayland-count 0 and negative values segfault at startup.

The creation loop in wlserver_init() is
for (int i = 0; i < g_nXWaylandCount; i++), so a count below 1 leaves
wlserver.wlr.xwayland_servers empty. main.cpp:1068 then does:

gamescope_xwayland_server_t *base_server = wlserver_get_xwayland_server(0);
setenv("DISPLAY", base_server->get_nested_display_name(), 1);

wlserver_get_xwayland_server() correctly returns NULL for an
out-of-range index, but the return value isn't checked here.

Reproduction

$ gamescope --xwayland-count 0 -- true
Segmentation fault (core dumped)     # exit 139

$ gamescope --xwayland-count -1 -- true
Segmentation fault (core dumped)     # exit 139

Backtrace:

#0  gamescope_xwayland_server_t::get_nested_display_name (this=0x0) at src/wlserver.cpp:3270
#1  main (...) at src/main.cpp:1070

Why validate instead of null-check

wlserver_get_xwayland_server(0) is used unchecked in main.cpp and in
roughly ten places in steamcompmgr.cpp (2573, 4413, 4436, 5582, 5992,
5997, 8783, ...), plus wlserver.cpp 367 and 872. Server 0 is assumed
to exist throughout, so guarding one call site would move the crash
rather than fix it. Rejecting the input at parse time matches the
existing parse_integer() and --backend error handling.

After

$ gamescope --xwayland-count 0 -- true
gamescope: invalid value for --xwayland-count, must be at least 1    # exit 1

Normal operation unaffected. meson test --suite gamescope passes 2/2.
Builds clean with --werror (689/689). Tested on fcc1341.

Independent of #, different file, different
mechanism and no dependency between them.