presence_dialoginfo: Fix memory leak in dlginfo_agg_nbody.

This commit fixes a couple of presence issues (introduced in 80c48e6):
 * There was a memory leak in dlginfo_agg_nbody.
 * Bounds checking in dlginfo_agg_nbody was off by 4 (potential stack
   corruption).
 * There were 2 PKG allocs, straight after one another while no alloc
   was needed at all: reduced to 1 optional one.
 * Replace "dumy" with "dummy" in the vicinity.
Merge pull request #174 from wdoekes/wjd-fix_dialogaggr_memleak

presence_dialoginfo: Fix memory leak in dlginfo_agg_nbody.