[Openais] new gcc warning uncovers nasty bug

[Jim Meyering]

Building with this bleeding edge gcc,

  gcc version 4.5.0 20090601 (experimental) (GCC)

I saw this new warning:

  totempg.c: In function 'totempg_groups_join':
  totempg.c:1038:23: warning: operation on 'instance->groups_cnt' may be undefined

Looking at that code, I see why.
FYI, this was introduced 11 months ago, with this change:
svn+ssh://svn.fedorahosted.org/svn/corosync/trunk@1582

Here's the fix: