tsan: add a useful debug check

llvm-svn: 214082

compiler-rt/lib/tsan/rtl/tsan_dense_alloc.h

@@ -65,6 +65,7 @@ class DenseSlabAlloc {
   }
 
   void Free(Cache *c, IndexT idx) {
+    DCHECK_NE(idx, 0);
     if (c->pos == Cache::kSize)
       Drain(c);
     c->cache[c->pos++] = idx;