MDEV-20562 btr_cur_open_at_rnd_pos() fails to return error for corrupted page

In mysql-server/commit@f46329044f8618212923bdf52e15d5b464201edc
the InnoDB function btr_cur_open_at_rnd_pos() was corrected so that
it would return a status that indicates whether the cursor was
successfully positioned. But this change was not correctly merged to
MariaDB in 2e814d47.

btr_cur_open_at_rnd_pos(): In the code path that was introduced in
MDEV-8588, properly return failure status.

No deterministic test case was found for this failure.
It was caught after removing the function
page_copy_rec_list_end_to_created_page() in a development branch.
As a result, the fill factor of index trees would improve, and
supposedly, so would the probability of btr_cur_open_at_rnd_pos()
reaching the intentionally corrupted page in the test
innodb.leaf_page_corrupted_during_recovery.
The wrong return value would cause
btr_estimate_number_of_different_key_vals() to wrongly invoke
btr_rec_get_externally_stored_len() on a non-leaf page and
trigger an assertion failure at the start of that function.

storage/innobase/btr/btr0cur.cc

@@ -2658,7 +2658,7 @@ btr_cur_open_at_rnd_pos_func(
 				index->table->file_unreadable = true;
 			}
 
-			goto exit_loop;
+			break;
 		}
 
 		page = buf_block_get_frame(block);
@@ -2815,12 +2815,11 @@ btr_cur_open_at_rnd_pos_func(
 		n_blocks++;
 	}
 
- exit_loop:
 	if (UNIV_LIKELY_NULL(heap)) {
 		mem_heap_free(heap);
 	}
 
-	return(true);
+	return err == DB_SUCCESS;
 }
 
 /*==================== B-TREE INSERT =========================*/