Fix incorrect pointer manipulation that caused mixmonitor recording to fail.

(closes ASTERISK-20834)
reported by Philippe Lindheimer
Patches:
	ASTERISK-20834.patch uploaded by Mark Michelson (License #5049)



git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/10-digiumphones@378655 f38db490-d61c-443f-a65b-d21fe96a405b