remove redundant assertion and min

We know several things here:

1. self.len <= self.cap, always
2. at <= self.len, asserted at the top of this method
3. after calling shallow_clone, other.cap == self.cap

Therefore, at <= self.len <= other.cap.

src/bytes_mut.rs

@@ -392,10 +392,8 @@ impl BytesMut {
 
  unsafe {
  let mut other = self.shallow_clone();
- assert!(at <= other.cap, "set_end out of bounds");
-
  other.cap = at;
- other.len = cmp::min(self.len, at);
+ other.len = at;
  self.set_start(at);
  other
  }