Add test for xor op generator

Test: th
Change-Id: Ibf41411e17c40f576ea6a5966110e107a42e29d9
diff --git a/payload_generator/extent_utils.h b/payload_generator/extent_utils.h
index 1cf7c77..2bd6626 100644
--- a/payload_generator/extent_utils.h
+++ b/payload_generator/extent_utils.h
@@ -131,7 +131,7 @@
 size_t GetNthBlock(const Container& extents, const size_t n) {
   size_t cur_block_count = 0;
   for (const auto& extent : extents) {
-    if (cur_block_count + extent.num_blocks() >= n) {
+    if (n - cur_block_count < extent.num_blocks()) {
       return extent.start_block() + (n - cur_block_count);
     }
     cur_block_count += extent.num_blocks();