Add validation in set_custom_mouse_cursor

Check if the image is valid.

platform/osx/os_osx.mm

@@ -1543,6 +1543,7 @@ void OS_OSX::set_custom_mouse_cursor(const RES &p_cursor, CursorShape p_shape, c
 
 		ERR_FAIL_COND(!texture.is_valid());
 		ERR_FAIL_COND(texture_size.width > 256 || texture_size.height > 256);
+		ERR_FAIL_COND(!image.is_valid());
 
 		image = texture->get_data();
 

platform/windows/os_windows.cpp

@@ -2074,6 +2074,7 @@ void OS_Windows::set_custom_mouse_cursor(const RES &p_cursor, CursorShape p_shap
 
 		ERR_FAIL_COND(!texture.is_valid());
 		ERR_FAIL_COND(texture_size.width > 256 || texture_size.height > 256);
+		ERR_FAIL_COND(!image.is_valid());
 
 		image = texture->get_data();
 

platform/x11/os_x11.cpp

@@ -2449,6 +2449,7 @@ void OS_X11::set_custom_mouse_cursor(const RES &p_cursor, CursorShape p_shape, c
 
 		ERR_FAIL_COND(!texture.is_valid());
 		ERR_FAIL_COND(texture_size.width > 256 || texture_size.height > 256);
+		ERR_FAIL_COND(!image.is_valid());
 
 		image = texture->get_data();