[RefBackend] Properly initialize refbackrt::Tensor refcount.

Although `refCount` is initialized as `std::atomic<int> refCount{0};` in
the definition of Tensor, our tail-allocating malloc would ignore it,
resulting in bogus values that led to leaks.

Caught with LeakSanitizer, but I added an assertion that the refcount is
non-negative to begin with, which should catch this bug in the future
fairly consistently (assuming the garbage refcount is negative half the
time).

include/npcomp/RefBackend/Runtime/UserAPI.h

@@ -34,6 +34,7 @@ public:
   // Creates a Ref and increments the refcount by 1.
   // rawPtr must be allocated with std::malloc.
   Ref(T *rawPtr) {
+    assert(rawPtr->refCount >= 0 && "expected non-negative refcount to start!");
     ptr = rawPtr;
     ptr->refCount += 1;
   }

lib/RefBackend/Runtime/Runtime.cpp

@@ -46,6 +46,7 @@ Tensor *Tensor::createRaw(ArrayRef<std::int32_t> extents, ElementType type,
   auto *tensor = static_cast<Tensor *>(
       std::malloc(sizeof(Tensor) + extents.size() * sizeof(std::int32_t)));
 
+  tensor->refCount.store(0);
   tensor->elementType = type;
   tensor->rank = extents.size();
   auto byteSize = getElementTypeByteSize(type) * totalElements(extents);