Fix a "thread.arena" mallctl bug.

Fix a variable reversal bug in mallctl("thread.arena", ...).
diff --git a/jemalloc/src/ctl.c b/jemalloc/src/ctl.c
index c37b4e7..1b28da4 100644
--- a/jemalloc/src/ctl.c
+++ b/jemalloc/src/ctl.c
@@ -1114,8 +1114,8 @@
 	unsigned newind, oldind;
 
 	newind = oldind = choose_arena()->ind;
-	WRITE(oldind, unsigned);
-	READ(newind, unsigned);
+	WRITE(newind, unsigned);
+	READ(oldind, unsigned);
 	if (newind != oldind) {
 		arena_t *arena;