Tweak BytesMut::remaining_mut

[stepancheg]

Same as in Vec: max allocation size in Rust is isize::MAX.

bytes/src/buf/buf_mut.rs

Lines 1601 to 1604 in 3e44f88

	fn remaining_mut(&self) -> usize {
	// A vector can never have more than isize::MAX bytes
	isize::MAX as usize - self.len()
	}