Fix fuzzer found null-deref in memory_quota (#27648)

src/core/lib/resource_quota/memory_quota.cc

@@ -376,7 +376,7 @@ void BasicMemoryQuota::Take(size_t amount) {
   auto prior = free_bytes_.fetch_sub(amount, std::memory_order_acq_rel);
   // If we push into overcommit, awake the reclaimer.
   if (prior >= 0 && prior < static_cast<intptr_t>(amount)) {
-    reclaimer_activity_->ForceWakeup();
+    if (reclaimer_activity_ != nullptr) reclaimer_activity_->ForceWakeup();
   }
 }
 

test/core/resource_quota/memory_quota_fuzzer_corpus/testcase-replenish-nullchk-after-creduce

@@ -0,0 +1,15 @@
+actions {
+  create_quota {}
+}
+actions {
+  create_allocator {}
+}
+actions{set_quota_size : 790} actions {
+  delete_quota {}
+}
+actions {
+  create_allocation {
+  max:
+    2
+  }
+}