binarystore: Cleanup optional config with default values.

Change-Id: If76d85f7aafbec30756dd01d871a78389d6e87a8
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/src/sys_file_impl.cpp b/src/sys_file_impl.cpp
index 9fab76a..7aadfba 100644
--- a/src/sys_file_impl.cpp
+++ b/src/sys_file_impl.cpp
@@ -25,7 +25,7 @@
     sys(sys)
 {
     fd_ = sys->open(path.c_str(), O_RDWR);
-    offset_ = offset ? *offset : 0;
+    offset_ = offset.value_or(0);
 
     if (fd_ < 0)
     {