posix: upgrade libevent from 1.4.7 to 1.4.13

third_party/libevent/min_heap.h

Index: third_party/libevent/min_heap.h
diff --git a/third_party/libevent/min_heap.h b/third_party/libevent/min_heap.h
index d47e5638ab7067f520f73418e75a59d53b8064d0..4fc83c01e76b867ecb0205d2cd122e1204e9e635 100644
--- a/third_party/libevent/min_heap.h
+++ b/third_party/libevent/min_heap.h
@@ -86,7 +86,17 @@ int min_heap_erase(min_heap_t* s, struct event* e)
 {
     if(((unsigned int)-1) != e->min_heap_idx)
     {
-        min_heap_shift_down_(s, e->min_heap_idx, s->p[--s->n]);
+        struct event *last = s->p[--s->n];
+        unsigned parent = (e->min_heap_idx - 1) / 2;
+	/* we replace e with the last element in the heap.  We might need to
+	   shift it upward if it is less than its parent, or downward if it is
+	   greater than one or both its children. Since the children are known
+	   to be less than the parent, it can't need to shift both up and
+	   down. */
+        if (e->min_heap_idx > 0 && min_heap_elem_greater(s->p[parent], last))
+             min_heap_shift_up_(s, e->min_heap_idx, last);
+        else
+             min_heap_shift_down_(s, e->min_heap_idx, last);
         e->min_heap_idx = -1;
         return 0;
     }