chan_sip: sip_subscribe_mwi_destroy should not call sip_destroy

sip_subscribe_mwi_destroy calls sip_destroy on the reference counted
mwi->call.  This results in the fields of mwi->call being freed, but
mwi->call itself it leaked.  If other code is still using mwi->call
it can cause problems.  This change uses dialog_unref instead, to
balance the ref provided by sip_alloc().

ASTERISK-24087 #close
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/3834/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@419440 f38db490-d61c-443f-a65b-d21fe96a405b