Strengthen debug double-free protections.

If a batch (currently always size()=1, though) would cause the span to become
free, we do not get an opportunity to cross-check other data like the free
object bitmap (>=64 byte objects for the 8KB page size).

PiperOrigin-RevId: 803632371
Change-Id: Ic8ea3b04f079afcd6ff94d3ab929653efe8bb63f

Author: ckennelly

tcmalloc/span.h

@@ -393,7 +393,7 @@ inline bool Span::FreelistPushBatch(absl::Span<void*> batch, size_t size,
                                     uint32_t reciprocal) {
   TC_ASSERT(!is_large_or_sampled());
   const auto allocated = allocated_.load(std::memory_order_relaxed);
-  TC_ASSERT_GT(allocated, 0);
+  TC_ASSERT_GE(allocated, batch.size());
   if (ABSL_PREDICT_FALSE(allocated == batch.size())) {
     return false;
   }