Material index of faces could be a invalid value after booleaning in
exact boolean in "Index Based" mode against objects with no material
slots, this is caused by the lack of initilization material remapping
array. Now all initilized to 0.

Assigning 0 explicitly in Fast mode as well for consistency.

@@ -378,6 +381,7 @@ static Array<short> get_material_remap_index_based(Object *dest_ob, Object *src_
     n = 1;
   }
   Array<short> remap(n);
+  remap.fill(0);

@@ -390,6 +394,7 @@ static Array<short> get_material_remap_transfer(Object &object,
 {
   const int material_num = mesh.totcol;
   Array<short> map(material_num);
+  map.fill(0);

@@ -375,7 +378,9 @@ static Array<short> get_material_remap_index_based(Object *dest_ob, Object *src_
 {
   int n = src_ob->totcol;

@@ -378,1 +381,3 @@
-    n = 1;
+    Array<short> remap(1);
+    remap[0] = 0;
+    return remap;