libsync: Fix a double-free.

sync_file_info, the only caller of legacy_fence_info_to_sync_file_info,
unconditionally frees legacy_info after
legacy_fence_info_to_sync_file_info is called. So, if this calloc
fails, we'll end up freeing legacy_info twice.

Bug: 27101951
Test: mma. Static analyzer complaint about double-free is gone.
Change-Id: I43bf820af9aadf30cb8eabce57416f69a8fccf89