Fix test_filter_ipc for cleared supplementary groups

This should fix part of [https://github.com/zeromq/libzmq/issues/1129].

tests/test_filter_ipc.cpp

@@ -122,8 +122,8 @@ int main (void)
     // Get the group and supplimental groups of the process owner
     gid_t groups[100];
     int ngroups = getgroups(100, groups);
-    assert (ngroups != -1 && ngroups != 0);
-    gid_t group = getgid(), supgroup = groups[0], notgroup = groups[ngroups - 1] + 1;
+    assert (ngroups != -1);
+    gid_t group = getgid(), supgroup = group, notgroup = group + 1;
     for (int i = 0; i < ngroups; i++) {
         if (supgroup == group && group != groups[i])
             supgroup = groups[i];