Set sinks_ to NULL after deletion in LogDestination::DeleteLogDestinations

[yying]

LogDestination::DeleteLogDestinations deletes sinks_ but does not set the global to NULL.

Further calls to LogDestination::AddLogSink will incorrectly use the stale sinks_ pointer and potentially crash (https://github.com/google/glog/blob/master/src/logging.cc#L621).

This simple fix sets sinks_ to NULL after the sinks_ is deleted.

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[yying]

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