Removed undefined behaviour.

Solves issue #49 (closed) which. That was indeed an undefined behaviour although it was probably fine.

src/runner_doiact.h

@@ -958,8 +958,8 @@ void DOPAIR2(struct runner *r, struct cell *ci, struct cell *cj) {
struct engine *restrict e = r->e;
int pid, pjd, k, sid;
double rshift, shift[3] = {0.0, 0.0, 0.0};
-  struct entry *restrict sort_i, *restrict sort_j;
-  struct entry *restrict sortdt_i = NULL, *restrict sortdt_j = NULL;
+  struct entry *sort_i, *sort_j;
+  struct entry *sortdt_i = NULL, *sortdt_j = NULL;
int countdt_i = 0, countdt_j = 0;
struct part *restrict pi, *restrict pj, *restrict parts_i, *restrict parts_j;
double pix[3], pjx[3], di, dj;