Minor fix found in mem module (#2014)
Minor fix found in mem module (#2014)

* fix(mem): Utilize ALIGN_MASK as much as possible

to avoid check LV_ARCH_64 again and again and
correct the comment about MEM_UNIT

* fix(mem): Simplify _lv_memset implementation by SET8/SET32/REPEAT8 macro

* fix(mem): Handle the out of memory gracefully in _lv_mem_buf_get

don't modify the state in the failure

* fix(mem): Remove the unnecessary memset, memcpy and assignment
littlevgl/lvgl,littlevgl/lvgl,littlevgl/lvgl,littlevgl/lvgl