[VL] Fix crash when there are unreleased memory pools during terminta…

…ting a Velox task (#8243)

A quick fix for a crash issue introduced by #8223.

Error:

terminate called after throwing an instance of 'gluten::GlutenException'
  what():  Unreachable code

cpp/velox/memory/VeloxMemoryManager.cc

@@ -336,7 +336,8 @@ bool VeloxMemoryManager::tryDestructSafe() {
   // Velox memory manager considered safe to destruct when no alive pools.
   if (veloxMemoryManager_) {
     if (veloxMemoryManager_->numPools() > 3) {
-      GLUTEN_CHECK(false, "Unreachable code");
+      VLOG(2) << "Attempt to destruct VeloxMemoryManager failed because there are " << veloxMemoryManager_->numPools()
+              << " outstanding memory pools.";
       return false;
     }
     if (veloxMemoryManager_->numPools() == 3) {