fix(obj): fix crash during multiple screen loader

[forqzy]

fix(lv_obj_draw): Update lv_obj_draw.c to avoid crash during multiple screen loader

[FASTSHIFT]

Are there any examples of recurrence? Why pass a null pointer to lv_obj_get_ext_draw_size?

[kisvegabor]

Please send a code snippet to reproduce the original issue.

[lvgl-bot]

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