 fix(buffer): prevent possible buffer overflow in `grow_zerofill`
 fix(buffer): prevent possible buffer overflow in `grow_zerofill`

    in `maybe_reserve` we have such code
    ```
        let new = self.buf.capacity() - self.buf.len();
        unsafe { grow_zerofill(&mut self.buf, new) }
    ```
    the original behavior of `grow_zerofill` in such case cause
    rewriting the memory behind the allocated vector.

yazaddaruvala/hyper,hannobraun/hyper,hyperium/hyper,markuskobler/hyper,hyperium/hyper,nickgonzales/hyper,hannobraun/hyper,Twey/hyper,tshepang/hyper,hannobraun/hyper,tshepang/hyper,markuskobler/hyper,softprops/hyper,yazaddaruvala/hyper,Keats/hyper,Twey/hyper,golddranks/hyper,Keats/hyper,nickgonzales/hyper,markuskobler/hyper,golddranks/hyper,Keats/hyper,hyperium/hyper,nickgonzales/hyper,golddranks/hyper,tshepang/hyper,Twey/hyper