Updates: Don't zero slop until closure has been pushed

Ensure that the the free variables have been pushed to the update
remembered set before we zero the slop.

(cherry picked from commit 134f7599)

rts/Updates.h

@@ -49,7 +49,6 @@
     W_ bd;                                                      \
                                                                 \
     prim_write_barrier;                                         \
-    OVERWRITING_CLOSURE(p1);                                    \
     bd = Bdescr(p1);                                            \
     if (bdescr_gen_no(bd) != 0 :: bits16) {                     \
       IF_NONMOVING_WRITE_BARRIER_ENABLED {                      \
@@ -60,6 +59,7 @@
     } else {                                                    \
       TICK_UPD_NEW_IND();                                       \
     }                                                           \
+    OVERWRITING_CLOSURE(p1);                                    \
     StgInd_indirectee(p1) = p2;                                 \
     prim_write_barrier;                                         \
     SET_INFO(p1, stg_BLACKHOLE_info);                           \