[client] clipboard: try each clipboard until one initializes

Previously, main.c would segfault at runtime if clipboards were disabled
via cmake flags, as the clipboards array would be empty but still
indexed during initialization.

Co-authored-by: Quantum <quantum2048@gmail.com>

client/clipboards/Wayland/src/wayland.c

@@ -398,7 +398,6 @@ static bool wayland_cb_init(
 {
   if (wminfo->subsystem != SDL_SYSWM_WAYLAND)
   {
-    DEBUG_ERROR("wrong subsystem");
     return false;
   }
 

client/clipboards/X11/src/x11.c

@@ -71,7 +71,6 @@ static bool x11_cb_init(
   // final sanity check
   if (wminfo->subsystem != SDL_SYSWM_X11)
   {
-    DEBUG_ERROR("wrong subsystem");
     return false;
   }