Prevent duplicate closeCallback calls in case of GC ghosts

[PVince81]

Sometimes PHP 7 GC decides to call stream_close again even after the
PHP request has ended, and even if fclose was called before already.

Note: the ghost is even so ghostly that you can't see it with a debugger.
Observed while debugging owncloud/core#22370 (comment)

Basically with the log statements from owncloud/core#26735 I can see that fclose was called and also the callback was called as well. But still, PHP somehow calls it again.

After applying this fix, this specific scenario doesn't cause errors to be logged again.

@icewind1991

[coveralls]

Coverage increased (+0.01%) to 95.946% when pulling 668abfe on PVince81:stream_close_scare_away_ghosts into 0700f81 on icewind1991:master.

[coveralls]

Coverage increased (+0.01%) to 95.946% when pulling 668abfe on PVince81:stream_close_scare_away_ghosts into 0700f81 on icewind1991:master.

[icewind1991]

👍 good catch finally finding the cause of this issue

[PVince81]

This is only 30℅ of the cause. Thanks for merging.

[icewind1991]

Tagged 0.5.2 with this fix