prevent memory leak when writing before ready

[rluvaton]

while trying to add a callback function to flush in pino I got into a situation where I logged and flushed before the file was ready so the cb never got called

[rluvaton]

I just noticed that the branch name has nothing to do with the fix 😅

[rluvaton]

let's merge this so I can continue working on pino flush?

(don't forget hacktoberfest 😅 I need some shirts 😂 )

[rluvaton]

Can you please release that so pinojs/pino#1827 can be merged?