rustuv: Don't zero-out data on clones

When cloning a stream, the data is already guaranteed to be in a consistent
state, so there's no need to perform a zeroing. This prevents segfaults as seen
in #15231

Closes #15231
auto merge of #15252 : alexcrichton/rust/issue-15231, r=pcwalton

When cloning a stream, the data is already guaranteed to be in a consistent
state, so there's no need to perform a zeroing. This prevents segfaults as seen
in #15231

Closes #15231