fs_movefile_and_open: -ENOSPC when no destination branch available

The create policy can legitimately return success with an empty
destination list (e.g. every candidate rejected). Previously we
then dereferenced dst_branch[0] and faulted. Return -ENOSPC so the
original write that triggered the move propagates a sensible errno
to userspace.

src/fs_movefile_and_open.cpp

@@ -82,6 +82,8 @@ _movefile_and_open(const Policy::Create &createFunc_,
   rv = createFunc_(branches_,fusepath_,dst_branch);
   if(rv < 0)
     return rv;
+  if(dst_branch.empty())
+    return -ENOSPC;
 
   origfd_flags = fs::getfl(origfd_);
   if(origfd_flags < 0)