Fix the "epoch" mallctl.

Fix the "epoch" mallctl to update cached stats even if the passed in
epoch is 0.

ChangeLog

@@ -81,6 +81,8 @@ found in the git revision history:
   - Check for NULL pointer in malloc_usable_size().
   - Fix an off-by-one heap profile statistics bug that could be observed in
     interval- and growth-triggered heap profiles.
+  - Fix the "epoch" mallctl to update cached stats even if the passed in epoch
+    is 0.
   - Fix bin->runcur management to fix a layout policy bug.  This bug did not
     affect correctness.
   - Fix a bug in choose_arena_hard() that potentially caused more arenas to be

src/ctl.c

@@ -1004,9 +1004,8 @@ epoch_ctl(const size_t *mib, size_t miblen, void *oldp, size_t *oldlenp,
 	uint64_t newval;
 
 	malloc_mutex_lock(&ctl_mtx);
-	newval = 0;
 	WRITE(newval, uint64_t);
-	if (newval != 0)
+	if (newp != NULL)
 		ctl_refresh();
 	READ(ctl_epoch, uint64_t);