canvas engine: ++safety code

added null checking, no logical changes.

src/modules/evas/engines/software_generic/evas_engine.c

@@ -3975,6 +3975,7 @@ _smart_merge(Tilebuf *tb, Tilebuf_Rect *rects)
    box = region_rects(region);
    n = region_rects_num(region);
    merged = calloc(1, n * sizeof(Tilebuf_Rect));
+   if (merged) {
         j = 0;
 #if 1
         // regions sometimes produce box sets like:
@@ -4027,6 +4028,7 @@ _smart_merge(Tilebuf *tb, Tilebuf_Rect *rects)
              j++;
           }
 #endif
+   }
    region_free(region);
    rects = mergelist;