res_musiconhold: Fix reference leaks caused when reloading with REF_DEBUG set

Due to a faulty function for debugging reference decrementing, it was possible
to reduce the refcount on the wrong object if two moh classes of the same name
were in the moh class container.

(closes issue ASTERISK-22252)
Reported by: Walter Doekes
Patches:
    18_moh_debug_ref_patch.diff Uploaded by Jonathan Rose (license 6182)
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Merged revisions 398937 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 421777 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 421779 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 421788 from http://svn.asterisk.org/svn/asterisk/branches/13

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@421793 65c4cc65-6c06-0410-ace0-fbb531ad65f3