AP_Scripting: fix OSD example availability check

The OSD binding exists when OSD_ENABLED is true, but the underlying
backend may be nullptr at runtime (e.g., when SITL is not built with
SFML support). The simple "if not osd" check only verifies the binding
exists, not whether the backend is valid.

Add a pcall-based check that tests if a method call succeeds, which
properly detects when the backend is unavailable.

Author: andyp1per

libraries/AP_Scripting/examples/osd.lua

@@ -131,13 +131,21 @@ end
 ------------------------------------------------------------
 -- Check OSD availability
 ------------------------------------------------------------
--- The osd binding returns nil if no OSD backend is available
+-- The osd binding exists but may have no backend available at runtime
 -- (e.g., when SITL is not built with --enable-sfml --sitl-osd)
+-- We test availability by attempting a method call with pcall
 if not osd then
     gcs:send_text(6, "OSD example: osd not available")
     return
 end
 
+-- Test if the OSD backend is actually available by calling a method
+local osd_available = pcall(function() return osd:get_screen() end)
+if not osd_available then
+    gcs:send_text(6, "OSD example: osd not available")
+    return
+end
+
 ------------------------------------------------------------
 -- Main update function
 ------------------------------------------------------------