Panic-safety unsoundness in `SplitVec::extend_from_slice` (uninitialized read)

SplitVec::extendfromslice increments the logical length self.len before cloning the incoming elements into the reserved slots. If an element's Clone panics mid-fill, unwinding leaves self.len counting slots that were never initialized. A later safe read get, indexing, iter then reads one of those...