vec-iree: fix buffer unmap mechanism

Per https://github.com/google/iree/blob/main/iree/hal/buffer.h#L430,
only mapped buffer can be unmapped. Guard it with `iree_status_is_ok`
check.

Change-Id: Id997798b7e26a13eef8285100c78eb24df22d59c

samples/simple_vec_mul/simple_vec_mul.c

diff --git a/samples/simple_vec_mul/simple_vec_mul.c b/samples/simple_vec_mul/simple_vec_mul.c
index 1ef2188..4bd6dc3 100644
--- a/samples/simple_vec_mul/simple_vec_mul.c
+++ b/samples/simple_vec_mul/simple_vec_mul.c
@@ -139,8 +139,8 @@
   }
   if (iree_status_is_ok(result)) {
     result = check_output_data(&mapped_memory);
+    iree_hal_buffer_unmap_range(&mapped_memory);
   }
-  iree_hal_buffer_unmap_range(&mapped_memory);
 
   iree_vm_list_release(inputs);
   iree_vm_list_release(outputs);