Fix a bug in file size limitation code.

Fix a bug in file size limitation code.

This fixes a bug in stapio, which checks written data size and switches
new file when it exceeds a limit. The problem is that written-data-size
counter ignores the first written-data size when switching files. So,
actual file size always exceeds the limit. This changes stapio to
initialize written-data-size counter with the size of the data which
will be written in new file.