fix(gradient): assert before dividing by 0 (#3228)

Co-authored-by: pengyiqiang <pengyiqiang@xiaomi.com>

src/draw/sw/lv_draw_sw_gradient.c

@@ -324,6 +324,8 @@ LV_ATTRIBUTE_FAST_MEM lv_grad_color_t lv_gradient_calculate(const lv_grad_dsc_t
         }
     }
 
+    LV_ASSERT(d != 0);
+
     /*Then interpolate*/
     frac -= min;
     lv_opa_t mix = (frac * 255) / d;