Cleanup: correct logic for qsort callback

Adding the additional check is more of a "technical" correction,
for the callback to return 0 the sort implementation would have
to compare an element in the array with itself.

Co-authored-by: Zheng-Ze <zheng-ze@noreply.localhost>

Ref !137723

source/blender/editors/object/object_vgroup.cc

@@ -1849,10 +1849,15 @@ static int inv_cmp_mdef_vert_weights(const void *a1, const void *a2)
   if (dw1->weight > dw2->weight) {
     return -1;
   }
+
+  /* Compare address for stable sort algorithm. */
   if (&dw1 < &dw2) {
-    return 1; /* compare address for stable sort algorithm */
+    return 1;
   }
-  return -1;
+  if (&dw1 > &dw2) {
+    return -1;
+  }
+  return 0;
 }
 
 /* Used for limiting the number of influencing bones per vertex when exporting