Merge branch 'sub_subset_bug' into 'master'

Typo in sub_subset that leads to a crash in the isothermal sphere case.

This bug has been there forever. @jregan uncovered it today by running the isothermal sphere case which has a rather deep tree. 

Re-assign to me if happy.

See merge request !183

src/runner_doiact.h

@@ -2361,7 +2361,7 @@ void DOSUB_SUBSET(struct runner *r, struct cell *ci, struct part *parts,
             DOSUB_SUBSET(r, ci->progeny[7], parts, ind, count, cj->progeny[0],
                          -1, 0);
           if (ci->progeny[7] != NULL && cj->progeny[0] == sub)
-            DOSUB_SUBSET(r, ci->progeny[0], parts, ind, count, cj->progeny[7],
+            DOSUB_SUBSET(r, cj->progeny[0], parts, ind, count, ci->progeny[7],
                          -1, 0);
           break;