Add checking on fd_posix refcounts

src/core/iomgr/fd_posix.c

@@ -104,14 +104,17 @@ static void destroy(grpc_fd *fd) {
 }
 
 static void ref_by(grpc_fd *fd, int n) {
-  gpr_atm_no_barrier_fetch_add(&fd->refst, n);
+  GPR_ASSERT(gpr_atm_no_barrier_fetch_add(&fd->refst, n) > 0);
 }
 
 static void unref_by(grpc_fd *fd, int n) {
-  if (gpr_atm_full_fetch_add(&fd->refst, -n) == n) {
+  gpr_atm old = gpr_atm_full_fetch_add(&fd->refst, -n);
+  if (old == n) {
     grpc_iomgr_add_callback(fd->on_done, fd->on_done_user_data);
     freelist_fd(fd);
     grpc_iomgr_unref();
+  } else {
+    GPR_ASSERT(old > n);
   }
 }