malloc: Initiate tcache shutdown even without allocations [BZ #28028]

After commit 1e26d35193 ("malloc: Fix
tcache leak after thread destruction [BZ #22111]"),
tcache_shutting_down is still not early enough.  When we detach a
thread with no tcache allocated, tcache_shutting_down would still be
false.

Reviewed-by: DJ Delorie <dj@redhat.com>

malloc/malloc.c

@@ -3144,12 +3144,13 @@ tcache_thread_shutdown (void)
   int i;
   tcache_perthread_struct *tcache_tmp = tcache;
 
+  tcache_shutting_down = true;
+
   if (!tcache)
     return;
 
   /* Disable the tcache and prevent it from being reinitialized.  */
   tcache = NULL;
-  tcache_shutting_down = true;
 
   /* Free all of the entries and the tcache itself back to the arena
      heap for coalescing.  */