LockedPool: Fix LockedPool::free(nullptr) to be a no-op

It was documented as such, but nullptr is never an address within an
Arena, so it would fall through and trigger the std::runtime_error.

src/support/lockedpool.cpp

@@ -320,6 +320,11 @@ void* LockedPool::alloc(size_t size)
 
 void LockedPool::free(void *ptr)
 {
+    // Freeing the nullptr pointer is OK.
+    if (ptr == nullptr) {
+        return;
+    }
+
     std::lock_guard<std::mutex> lock(mutex);
     // TODO we can do better than this linear search by keeping a map of arena
     // extents to arena, and looking up the address.

src/test/allocator_tests.cpp

@@ -124,6 +124,9 @@ BOOST_AUTO_TEST_CASE(arena_tests)
 
     BOOST_CHECK(b.stats().total == synth_size);
     BOOST_CHECK(b.stats().free == synth_size);
+
+    // Check that Arena::free may be called on nullptr.
+    b.free(nullptr);
 }
 
 /** Mock LockedPageAllocator for testing */
@@ -229,6 +232,9 @@ BOOST_AUTO_TEST_CASE(lockedpool_tests_live)
     BOOST_CHECK(pool.stats().total <= (initial.total + LockedPool::ARENA_SIZE));
     // Usage must be back to where it started
     BOOST_CHECK(pool.stats().used == initial.used);
+
+    // Check that LockedPool::free may be called on nullptr.
+    pool.free(nullptr);
 }
 
 BOOST_AUTO_TEST_SUITE_END()