don't open log file before trancate file

failed to trancate file while file has opened on some filesystem(on the win32), don't open log file before trancate file.

I hope influxdb can be ported to win32 platform, I have more pull request, which is one of。

 #839 is duplcate, I close it
While this is an admirable programming style to prevent the classic bug of assignment instead of comparison, it is not possible in Go to make this mistake. The compiler won't let you do it.

http://play.golang.org/p/c4Em6G0NtD
http://play.golang.org/p/YoYqojbVU5

I suggest you follow the standard style.