Fix bugs in BytesMut::reserve_inner (#544)

src/bytes_mut.rs

@@ -646,7 +646,7 @@ impl BytesMut {
                     self.cap = v.capacity();
                 } else {
                     // calculate offset
-                    let off = v.capacity() - self.cap;
+                    let off = (self.ptr.as_ptr() as usize) - (v.as_ptr() as usize);
 
                     // new_cap is calculated in terms of `BytesMut`, not the underlying
                     // `Vec`, so it does not take the offset into account.