LockedPool: fix explosion for illegal-sized alloc

Check for unreasonable alloc size in LockedPool rather than lancing through new
Arenas until we improbably find one worthy of the quixotic request or the system
can support no more Arenas.

src/support/lockedpool.cpp

@@ -276,6 +276,11 @@ LockedPool::~LockedPool()
 void* LockedPool::alloc(size_t size)
 {
     std::lock_guard<std::mutex> lock(mutex);
+
+    // Don't handle impossible sizes
+    if (size == 0 || size > ARENA_SIZE)
+        return nullptr;
+
     // Try allocating from each current arena
     for (auto &arena: arenas) {
         void *addr = arena.alloc(size);