[riot-notifications] [RIOT] sys/xtimer: Avoid race incrementing multiple periods in _timer_callback (#3998)

[Martine Lenders]

There needs to be a PR to the release branch too for this, right?

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Reply to this email directly or view it on GitHub:
https://github.com/RIOT-OS/RIOT/pull/3998#issuecomment-144332496
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