res_musiconhold: Fix MOH restarting where it left off from the last hold.

Restore code removed by https://reviewboard.asterisk.org/r/3536/ that
introduced a regression that prevents MOH from restarting were it left off
the last time.

ASTERISK-24019 #close
Reported by: Jason Richards
Patches:
      jira_asterisk_24019_v1.8.patch (license #5621) patch uploaded by rmudgett

Review: https://reviewboard.asterisk.org/r/3928/
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Merged revisions 421976 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 421977 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 421978 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@421979 f38db490-d61c-443f-a65b-d21fe96a405b
res_musiconhold.c: Release any format refs before memset().

* Clear the channel music_state pointer before destroying the music_state
object for safety.


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@422037 f38db490-d61c-443f-a65b-d21fe96a405b