Add an extra assert to internal buffers in futures/streams (#13103)

This is effectively already implied by the rest of the implementation
but by making this a first-class assert it makes it easier to verify the
subsequent `unsafe` code is correct.

Author: alexcrichton

crates/wasmtime/src/runtime/component/concurrent/futures_and_streams/buffers.rs

@@ -223,6 +223,7 @@ pub struct SliceBuffer {
 
 impl SliceBuffer {
     pub fn new(buffer: Vec<u8>, offset: usize, limit: usize) -> Self {
+        assert!(offset <= limit);
         assert!(limit <= buffer.len());
         Self {
             buffer,