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
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Merged revisions 393627 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 393628 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@393629 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_dahdi.c

@@ -15617,6 +15617,7 @@ static int dahdi_restart(void)
 	dahdi_softhangup_all();
 	ast_verb(4, "Final softhangup of all DAHDI channels complete.\n");
 	destroy_all_channels();
+	memset(round_robin, 0, sizeof(round_robin));
 	ast_debug(1, "Channels destroyed. Now re-reading config. %d active channels remaining.\n", ast_active_channels());
 
 	ast_mutex_unlock(&monlock);