chan_dahdi: Fix segfault reloading chan_dahdi when round robin is used.

* Clear round_robin[] in dahdi_restart().

(closes issue ASTERISK-21847)
Reported by: Ivo Andonov
Patches:
      jira_asterisk_21847_v1.8.patch (license #5621) patch uploaded by rmudgett


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@393627 f38db490-d61c-443f-a65b-d21fe96a405b